# Lakhmir Singh Class 10 Chemistry 5th Chapter Periodic Classification Of Elements Solution

## Lakhmir Singh Class 10 Chemistry 5th Chapter Periodic Classification Of Elements Solution

Lakhmir Singh Manjit Kaur Chemistry Solution: Periodic Classification Of Elements Chapter 5. Here you get easy Solutions of Lakhmir Singh Class 10 Chemistry Solution Chapter 5. Here we have given Chapter 5 all Solution of Class 10. Its help you to complete your homework.

• Board – CBSE
• Text Book – Chemistry
• Class – 10
• Chapter – 05

### Lakhmir Singh Class 10 Chemistry 5th Chapter Solution

1. (a) On what basis did Mendeleev arrange the elements in his periodic table?

Answer: – on the basis of the atomic masses Mendeleev arranged the elements in
his periodic table.

(b) On what basis are they arranged now?

Answer: – on the basis of atomic numbers the elements in the modern periodic
table are arranged now.

2.) State whether the following statements are true or false:

(a) Newlands divided the elements into horizontal rows of eight elements each.

Newland divided the elements into horizontal rows of SEVEN elements each.

(b) According to Mendeleev’s periodic law, the properties of elements are a periodic function of their atomic numbers.

According to Mendeleev’s periodic law, the properties of elements are a periodic
function of their atomic masses.

(c) The elements in a group have consecutive atomic numbers.

The elements in a PERIOD have consecutive atomic numbers.

3. Name the Russian chemist who said that the properties of elements are a periodic function of their atomic masses.

Answer: – Mendeleev was the Russian chemist who said the properties of the elements are a periodic function of their atomic masses.

4. Rewrite the following statements after correction, if necessary:

(a) Groups have elements with consecutive atomic numbers.

Answer: – Periods have the elements with consecutive atomic numbers.

(b) Periods are the horizontal rows of elements.

Answer: – Above sentence is correct.

(c) Isotopes are the elements of the same group.

Answer: – Above sentence is correct.

5. Name the scientists who gave the following laws in the early classification of elements:

(a) Law of octaves

Answer: – Newland gave the law of octave in the early classification of elements.

Answer: – Dobereiner gave the law of triads in the early classification of elements.

6.) A, B and C are the elements of a Dobereiner’s triad. If the atomic mass of A is 7 and that of C is 39, what should be the atomic mass of B?

According to the Dobereiner’s triad the elements arranged in the increasing atomic masses, the mean of the atomic masses of the first and the third element is the approximate atomic mass of the second element.

Then as per the law of the Dobereiner’s triad – Atomic masses of A = 7, C = 39

Then Therefore, the atomic mass of the element B is 23.

7. X and Y are the two elements having similar properties which obey Newlands’ law of octaves. How many elements are there in-between X and Y ?

Answer: – there should be 6 elements in between the X and Y because, Newlands
octave contains 8 elements.

8. What was the Mendeleev’s basis for the classification of elements?

Answer: – Mendeleev classified the elements on the basis of the atomic masses.

9. In the classification of the known elements, Mendeleev was guided by two factors. What are those two factors ?

Answer: – Mendeleev was guided up by two factors which were the increasing
atomic masses and the second one is arranging elements with the same
properties together.

10. Name two elements whose properties were predicted on the basis of their
positions in Mendeleev’s periodic table.

Answer: – Scandium and the gallium are the two elements which properties were
predicted on the basis of their position in the Mendeleev periodic table.

11.The three elements predicted by Mendeleev from the gaps in his periodic table were known as eka-boron, eka- aluminium and eka-silicon. What names were given to these elements when they were discovered later on ?

Answer: – the names were given to those elements later on are –

eka-boron=Scandium

eka-aluminium= Gallium

eka-silicon= Germanium

12.Name two elements whose properties were predicted on the basis of their
positions in Mendeleev’s periodic table.

Answer: – Scandium and the gallium are the two elements which properties were
predicted on the basis of their position in the Mendeleev periodic table.

13. State one example of a Dobereiner’s triad, showing in it that the atomic mass of middle element is half-way between those of the other two.

The example of the Dobereiner’s triad is Lithium, Sodium, and Potassium in which
the middle element sodium has the atomic mass 23 which is the mean of the
other two elements which are lithium 7 and potassium 39.

14.Which group of elements could be placed in Mendeleev’s periodic table
later on, without disturbing the original order? Give reason.

Answer: – Noble gases are the group 18 elements which were placed in the modern periodic table later on with no disturbing the original arrangement. Noble gases have completely filled valence shell with electrons thus the noble gases are inert and hence they do not disturb the original arrangement.

15.Fill in the following blanks with suitable words :

(a) The basis for modern periodic table is ……….

Answer: – The basis for modern periodic table is atomic numbers.

(b) The horizontal rows in a periodic table are called ……….

Answer: – The horizontal rows in a periodic table are called periods.

(c) Group 1 elements are called ……….

Answer: – Group 1 elements are called alkali metals.

(d) Group 17 elements are known as ……….

Answer: – Group 17 elements are known as halogens.

(e) Group 18 elements are called ……….

Answer: – Group 18 elements are called noble gases.

(f) According to Newlands’ classification of elements, the properties of sulphur are similar to those of oxygen because sulphur is the ………. element starting from oxygen.

Answer: – According to Newlands’ classification of elements, the properties of
sulphur are similar to those of oxygen because sulphur is the eighth element
starting from oxygen.

16.

(a) What is meant by (i) a group, and (ii) a period, in a periodic table?

Group:- groups are the vertical row of the periodic table in which the elements are arranged as per the similar electronic configuration of the elements.

Period:- periods are the horizontal rows of the periodic table in which the
elements are arranged as per the increasing atomic numbers.

(b) How many periods and groups are there in the long form of periodic table?

Answer: – there are 18 groups and 7 periods in the modern periodic table.

(c) Give two examples each of (i) group 1 elements (ii) group 17 elements (iii) mgroup 18 elements.
Answer: – the examples of the each group are as follows-
(i) group 1 elements – lithium and sodium
(ii) group 17 elements – fluorine and bromine
(iii) Group 18 elements- helium and argon

17. (a) In the modern periodic table, which are the metals among the first ten elements?

Answer: – in the modern periodic table the beryllium and lithium are the metals among the first ten elements.

(b) What is the significance of atomic number in the modern classification
of elements? Explain with the help of an example.
The significance of the atomic number in the modern periodic table is that the
elements are classified on the basis of the increasing atomic numbers. Hydrogen
with atomic number 1, helium with 2, lithium with 3, beryllium with 4 are
arranged in the increasing atomic number in the modern periodic table. Elements
with the same chemical properties and the same number of valance electrons are
placed in the same group.

18. (a) How were the positions of isotopes of an element decided in the modern periodic table?

Answer: – the elements in the modern periodic table are arranged as per the
increasing order of the atomic numbers; the Isotopes are the elements which
have same atomic number but different atomic mass thus the Isotopes are not
placed separately in the modern periodic table.

(b) How were the positions of cobalt and nickel resolved in the modern periodic table?

Answer: – Cobalt is placed before the Nickel in the modern periodic table because
cobalt has atomic number 27 and the Nickel has atomic number 28, and in the
modern periodic table elements are arranged on the basis of the increasing
atomic number.

(c) Where hydrogen should be placed in the modern periodic table? Give

Answer: – hydrogen has atomic number 1 and it is placed correctly in the modern
periodic table on the first number among all the elements.

19. (a) On which side of the periodic table will you find metals?

Answer: – on left side of the periodic table we will find metals.

(b) On which side of the periodic table will you find non-metals ?

Answer: – on the right side of the periodic table we will find Non – metals.

(c) What is the name of those elements which divide metals and non-metals in the periodic table?

Answer: – the metalloids are the elements which divide the metals and the non-metals in the modern periodic table.

20. (a) Name three elements that have a single electron in their outermost shells.

Answer: – the three elements which have a single electron in their outermost
shells are sodium, lithium and potassium.

(b) Name two elements that have two electrons in their outermost shells.

Answer: – the two electrons which have two electrons in their outermost shells
are calcium and magnesium.

(c) Name three elements with completely filled outermost shells.

Answer: -three elements which have completely filled outermost shells are Neon,
Argon, and Krypton.

21.) What is Debereiner’s law of triads ? Explain with the help of one example of

In the dobereiner’s triads three elements are arranged as per the increasing
atomic masses of the elements, and the mean of the atomic masses of the first
and last element is the approximate value of atomic mass of the middle elements.
The example of the Debereiner’s law of triads is –
Lithium, Sodium, and Potassium

Atomic masses of them are –

Lithium – 7

Sodium – 23

Potassium – 39

The atomic mass of the sodium is the mean value of the other two elements.

22.) What is Newlands’ law of octaves ? Explain with an example.

Newland’s law of octave states that the elements are arranged as per the increasing order of the atomic mass in which the properties of first element have the same properties as like the 8th element.

The example of the Newland’s law of octave is –

• H , Li, Be, B , C, N, O
• F, Na, Mg, Al, Si, P,S
• Cl, K, Ca, Cr, Ti, Mn, Fe

23.)
(a) Did Dobereiner’s triads also exist in the columns of Newlands’ law of octaves? Explain your answer.

Answer: – Yes, dobereiner’s triads is also exists in the column of the Newland’s law of
octaves. The example of these is – Li, Na, and K which follows the rule of the dobereiner’s triads.

(b) What were the limitations of Dobereiner’s classification of elements?

Answer: -the limitations of the Dobereiner’s classification of elements are as
follows –

• 1) The rule is not obeyed by most of the elements.
• 2) Newly added elements in the periodic table were not applicable by the rule.
• 3) Up till now only 5 dobereiner’s triads have been found.

(c) What were the limitations of Newlands’ law of octave?

Answer: – the limitations of the Newland’s law of octave are as follows –

• 1) Newland adjust two elements at the same place.
• 2) The rule was applicable only for lighter elements.
• 3) The rule was obeyed only upto the calcium. i.e. atomic number 40.
• 4) Only 63 elements were considered as per the Newland’s octave law.

24. (a) State the periodic law on which Mendeleev’s periodic table was based. Why and how was this periodic law changed ?

Answer: – the periodic law on which the Mendeleev’s periodic table was based is
that, the atomic masses are periodic function of the elements. And the table is
based on the increasing atomic masses of the elements. But the Mendeleev
periodic table has many drawbacks, it does not state the position of the newly
found elements and thus the law changed and in the modern periodic table the
new law formed that was the elements were arranged on the basis of their
atomic numbers.

(b) Explain why, the noble gases are placed in a separate group.

Answer: – noble gases are also known as inert gases. Their outermost valence
shell is completely filled. And hence these are placed in a separated group on the
basis of their completely filled valance shell electrons.

25. (a) State the merits of Mendeleev’s classification of elements.

1) Mendeleev’s periodic table predicted the properties of the elements on the basis of their position in the periodic table.

2) Mendeleev’s periodic table predicted that there is existence of some elements which was further named as scandium, gallium, and germanium.

(b) Describe two anomalies of Mendeleev’s periodic classification of elements.

1) Mendeleev’s periodic table could not predict where the isotopes should place in the periodic table.

2) Some elements with same properties were placed in the different group while the different properties elements were placed in the same group.

3) Couldn’t predict the position of some elements correctly which
have multiple properties like- Hydrogen.

26.
(a) How do the properties of eka-aluminium element predicted by Mendeleev

Answer: – at the early stage on the basis of the Mendeleev law; he stated that
there are some more elements whose properties were predicted by the
Mendeleev. One of them he gave a name Eka – aluminum. Later on when the
gallium discovered then the properties of the Eka- aluminum and the gallium
were almost same with each other. Both the elements were showing the same
properties like atomic mass, formula, melting and boiling point etc. hence it is
confirmed that both the elements are same.

(b) What names were given by Mendeleev to the then undiscovered elements

(i) scandium

(ii) gallium

(iii) germanium

27.

(a) Why do we classify elements?

Answer: – To properly organize all the different types of elements according to
their properties we have classified the element in to suitable tables which have
groups and periods so that the information of the element found easily.

(b) What were the two criteria used by Mendeleev to classify the elements in
his periodic table?

1) The first criteria used by the Mendeleev to classify the elements were according to the atomic masses of the elements.

2) The second criteria of the Mendeleev to classify the elements were the similar chemical properties of the elements.

(c) Why did Mendeleev leave some gaps in his periodic table ?

Answer: – at the time of the Mendeleev periodic table some elements were not known. But he predicted that the elements are there according to the properties of that group and thus he left gaps in between the periodic table.

(d) In Mendeleev’s periodic table, why was there no mention of noble gases
like helium, neon and argon?

Answer: – at the time of the Mendeleev periodic table the noble gases or inert
gases like helium, argon, neon and krypton were not known or not found. Thus
the inert gases were not mention in the periodic table.

(e) Would you place the two isotopes of chlorine, Cl-35 and Cl-37 in different
slots because of their different atomic masses or in the same slot because

Answer: – I will place both the isotopes of the chlorine Cl- 35 and Cl- 37 in the
same slots because of their same chemical properties also they possess the same
atomic numbers.

28.

a.) State Mendeleev’s periodic law.

Answer: – The Mendeleev periodic law states that the chemical and the physical
properties of the elements are have the periodic functions which are their atomic
masses.

b.) What chemical properties of elements were used by Mendeleev in creating his periodic table?

Answer: – The basic chemical properties were used by the Mendeleev in creating
the periodic table was the formula of the hydrides and the oxides which are
produces by the elements.

C.) State any three limitations of Mendeleev’s classification of elements.

1) Mendeleev’s periodic table could not predict where the isotopes should place in the periodic table.

2) Some elements with same properties were placed in the different group while the different properties elements were placed in the same group.

3) Couldn’t predict the position of some elements correctly which have multiple properties like- Hydrogen.

4) He couldn’t explain the wrong order of the atomic masses in the periodic table.

d.) Besides gallium, which two other elements have since been discovered for which Mendeleev had left gaps in his periodic table?

Answer: – beside the gallium the other two elements have since been discovered
for which the Mendeleev left gaps in the periodic table were the scandium – Eka – Boron and germanium – Eka-silicon.

e.) Which group of elements was missing from Mendeleev’s original periodic table?

Answer: – the inert gases group of elements was missing from the Mendeleev’s

29.

(a) State modern periodic law.

Answer: – the modern periodic law states that – the physical and the chemical
properties of the elements are the periodic functions of the atomic numbers of
the elements.

(b) How does the electronic configuration of the atom of an element relate
to its position in the modern periodic table?

Answer: – as the electronic configuration of the elements states that how many
valance electrons are there in the outermost shell, and according to that we can
find out the position of the elements in the groups and on the basis of hoe much
valance shells are there we can find out the position of the elements in the
periods.

(c) How could the modern periodic law remove various anomalies of
Mendeleev’s periodic table? Explain with examples.

Answer: – Various drawbacks of the Mendeleev periodic table were removed by
the modern periodic table as follows –

1) Hydrogen is placed at the position first according to the atomic number 1.

2) Isotopes are placed at the same box due to their same atomic number also the same properties.

3) Some elements position wrongly ordered due to their atomic masses, but in the modern periodic table it is correctly placed according to the atomic
numbers.

(d) Is it possible to have an element having atomic number 1.5 placed between hydrogen and helium

Answer: – yes, it is possible to placed an element having an atomic number 1.5
between the hydrogen 1 and helium 2 in the modern periodic table, because the
modern periodic table is based on the increasing order of the atomic numbers of
the elements.

(e) Name the scientist who prepared modern periodic table.

Answer: – the concept of the modern periodic table was prepared by the Henry
Moseley. He stated the modern periodic law. But the modern periodic table was
prepared by the Neils Bohr.

Multiple Choice Questions (MCQs)

30.) In Mendeleev’s periodic table, gap was not left for one of the following elements. This element is :

(a) gallium (b) beryllium (c) germanium (d) scandium

Explanation – In the modern periodic table in the basis of the properties of the
elements Mendeleev predicated some elements and left gaps for gallium,
germanium and scandium but for the beryllium the gap was not left because he
couldn’t predict occurrence of the element beryllium.

31.) The Newlands’ law of octaves for the classification of elements was found to be applicable only up to the element :

(a) potassium (b) calcium (c) cobalt (d) phosphorus

Explanation – The Newlands’ law of octaves for the classification of elements was
found to be applicable only up to the element calcium- 40 because, after that the
elements does not obey the rules of the Newland’s octave.

32.) According to Mendeleev’s periodic law, the elements were arranged in the
periodic table in the order of :

(a) decreasing atomic numbers

(b) increasing atomic numbers

(c) decreasing atomic masses

(d) increasing atomic masses

Answer: – (d) increasing atomic masses

Explanation – Mendeleev arranged the periodic table on the basis of the
increasing atomic masses and properties of the elements.

33.) The three elements having chemical symbols of Si, B and Ge are :

(a) all metals

(b) all non-metals

(c) all metalloids

(d) Si is metalloid, B is metal and Ge is non-metal

Explanation – All the given elements Silicon, boron and germanium are metalloids
and thus these are placed diagonally in the periodic table.

34. In Mendeleev’s periodic table, gaps were left for the elements to be discovered later on. An element which found a vacant place in the periodic table later on is :

(a) Be (b) Si (c) Ge (d) Se

Explanation -Germanium is the element for which the gap was left in the
Mendeleev periodic table. Mendeleev predicated the element on the basis of the
periodic properties of the groups and he gave the element name which is Ekasilicon.

35. The three imaginary elements X, Y and Z represent a Dobernener’s triad. If the atomic mass of element X is 14 and that of element Y is 46, then the atomic mass of element Z will be :

(a) 28 (b) 60 (c) 78 (d) 72

solution = Therefore, the atomic mass of the element Z is 78.

36. The atomic numbers of four elements P, Q, R and S are 6, 8, 14 and 16
respectively. Out of these, the element known as metalloid is:

(a) P (b) Q (c) R (d) S

Explanation – The atomic 14 is representing the element silicon which is a
metalloid. Other elements 6, 8 and 16 are not metalloids. Hence R is the
metalloid.

37. Which of the following statement is correct in regard to the classification of elements ?

(a) Elements in modern periodic table are arranged on the basis of increasing
atomic masses.

(b) Elements in Mendeleev’s periodic table are arranged on the basis of increasing
atomic numbers

(c) In modern periodic table, the element nickel of lower atomic mass is kept
before the element cobalt of higher atomic mass.

(d) In modern periodic table, the isotopes of chlorine having different atomic
masses are kept in the same group.

Answer: – (d) In modern periodic table, the isotopes of chlorine having different
atomic masses are kept in the same group

Explanation – Above statement is correct regarding the classification of the
element.The other given statements are wrong according to the periodic classification.

38. Which of the following statement about the modern periodic table is correct ?

(a) It has 18 horizontal rows known as periods.

(b) It has 7 vertical columns known
as periods.

(c) It has 18 vertical columns known as groups.

(d) It has 7 horizontal rows known as groups.

Answer: -(c) It has 18 vertical columns known as groups.

Explanation – Modern periodic table contains 18 vertical columns and 7 horizontal rows.

39. An element X forms an oxide X2O3. In which group of Mendeleev’s periodic table is this element placed ?

(a) group II (b) group III (c) group V (d) group VIII

Explanation- element X will be placed in the group 3 because it forms an oxides
X2O3 and it has 3 electrons in its valance shell orbital.

40. The modern periodic table was prepared by :

(a) Dobereiner (b) Newlands (c) Bohr (d) Mendeleev

Explanation- the concept of modern periodic table was given by the Henry but
the table was formed by Neils bohr.

41. The atomic particle whose number in the atoms of an element always remains
the same and which forms the real basis for the modern classification of elements is :

(a) electron (b) proton (c) neutron (d) meson

Explanation- the number of proton does not change in the atoms of the element.
Number of electron and the neutron changes.

##### PART – 2

1. Given alongside is a part of the periodic table :

As we move horizontally from left to right :

(i) What happens to the metallic character of the elements ?

Answer: – when we move from left to right in the periodic table the metallic characters of the elements decreases.

(ii) What happens to the atomic size ?

Answer: – when we move from left to right in the periodic table the atomic size of
the elements decreases.

2. How would the tendency to gain electrons change on moving from left to right
in a period of the periodic table ?

Moving from left to right in the periodic table metallic characters and atomic size
decreases and hence effective nuclear charge on the electrons increases thus the
tendency to gain an electron charge also increases.

3. How would the tendency to lose electrons change as we go from left to right
across a period of the periodic table ?

Moving from left to right in the periodic table atomic size decreases and hence
force of nuclear attraction also decreases with it, thus the tendency to lose an
electron also decreases in the periods of the periodic table.

4.
(a) How does the chemical reactivity of alkali metals vary on going down in
group 1 of the periodic table ?

Answer: – on going down in group 1 of the periodic table the chemical reactivity of
alkali metals increases.

(b) How does the chemical reactivity of the halogens vary on going down in
group 17 of the periodic table ?

Answer: – on going down in group 17 of the periodic table the chemical reactivity
of the halogens decreases.

5.) What property do all elements in the same column of the periodic table as
boron have in common ?

Answer: – the property all elements same column of the periodic table as boron
have common is that every element has 3 electrons in its valance shell orbitals.

6.) What property do all the elements in the same group of the periodic table as
fluorine have in common ?

Answer: – the property all elements same column of the periodic table as fluorine
have common is that every element has 7electrons in its valance shell orbitals.

7.

(a) What is the number of valence electrons in the atoms of first element in a
period ?

Answer: -The number of valence electron in the atoms of first elements hydrogen in a
period is 1. As the 1st element hydrogen has only one electron and it has the atomic
number 1. i.e. only 1 electron which is present in its outermost shell.

(b) What is the usual number of valence electrons in the atoms of the last
element in a period?

Answer: – the usual numbers of valence electrons in the atoms of the last element
in a period are 8.

8. State whether the following statement is true or false :

On going down in a group of the periodic table, the number of valence electrons increases.

On going down in a group of the periodic table, the number of valence electrons
does not increase it remains as it is in the number.

9. What is the major characteristic of the first elements in the periods of the
periodic table? What is the general name of such elements?

The major characteristic of the first elements in the periods of the periodic table
is that there is only one electron in its valance shell. The general names of such
elements are alkali metals.

10. How do the atomic radii of elements change as we go from left to right in a
period of the periodic table?

Answer: – usually the atomic radii of the elements decrease as we move from left
to right in the period of the periodic table.

11.) What happens to the metallic character of the elements as we go down in a
group of the periodic table?

Answer: – as we move down in a group of the periodic table the metallic characters increase.

12. How does the number of valence electrons vary on moving from left to right :

(i) in the first period of the periodic table ?

Answer: – it increases with addition of one by one electron in the outermost valance shell. It goes from 1 to 7.

(ii) in the second period of the periodic table ?

Answer: – it increases with addition of one by one electron in the outermost
valance shell. It goes from 1 to 7.

13. How does the valency of elements change on moving from left to right in the
third period of the periodic table ?

Answer: – in the third period of the periodic table of elements the valency of the elements increases from 1 to 4 and then decreases to the 0.

14. How does the valency of elements vary in going down a group of the periodic
table ?

Answer: – valency does not vary going down the periodic table it remains as it is.

15.) Name the element which is in :

(a) first group and third period.

Answer: – sodium ( Na) is present in the 1st group and 3rd period of the periodic table.

(b) seventeenth group and second period.

Answer: – Fluorine (F) is the element which is present in the 17th group and the 7th
period of the periodic table.

16.) How do electronic configurations of elements change in second period of
periodic table with increase in atomic numbers ?

Answer: -the electronic configuration of elements change in the second period of
the periodic table as the number of valance electrons increases but the number of
shell remains constant moving from left to right in the period.

17.) Arrange the following elements in increasing order of their atomic radii :
Li, Be, F, N

Answer: – the correct arrangement of the elements in the increasing order of their
atomic radii is as follows- F < N < Be < Li

18.) Arrange the following elements in the increasing order of their metallic
character :

Mg, Ca, K, Ga

Answer: – the correct arrangement of the elements in the increasing order of their
metallic character is as follows – Ga < Mg< Ca< K

19. Rewrite the following statements after correction, if necessary :

(i) Elements in the same period have equal valency

Answer: – Elements in the same groups have equal valency

(ii) The metallic character of elements in a period increases gradually on moving from left to right.

Answer: -The metallic character of elements in a period decreases gradually on moving from left to right.

20. Fill in the blanks in the following statements :

(a) The horizontal rows in a periodic table are called ……….

Answer: – The horizontal rows in a periodic table are called periods.

(b) In going across a period (right to left) in periodic table, the atomic size of
the atom ……….

Answer: – In going across period (right to left) in periodic table, the atomic size of
the atom increases.

(c) On moving from right to left in the second period, the number of valence
electrons ……….

Answer: – On moving from right to left in the second period, the number of
valence electrons decreases.

(d) On going down in a group in the periodic table, the metallic character of
elements ………

Answer: – On going down in a group in the periodic table, the metallic character of
elements increases.

(e) The tendency to gain an electron ………. on moving down in a group of the
periodic table.
Answer: – The tendency to gain an electron decreases on moving down in a group
of the periodic table.

21. Nitrogen (atomic number 7) and phosphorus (atomic number 15) belong to group 15 of the periodic table. Write the electronic configurations of these two elements. Which of these will be more electronegative? Why?

Answer: – The electronic configurations of the elements are as follows-

1) Nitrogen (N) 7 = (2, 5)

2) Phosphorous (P) = (2,8,5)

Among the 2 elements the more electronegative element will be nitrogen
because the atomic size of the nitrogen is small than the phosphorus and due to
that the electrons are more attracted towards the nucleus of the atom.

22. An element X belongs to group 2 and another element Y belongs to group 15
of the periodic table: Explain how you have arrived at your answers.

(a) What is the number of valence electrons in X?

Answer: – the number of valance electrons of the element X is 2, because it beling
to the group II.

(b) What is the valency of X?

Answer: – the valancy of the X is 2 because it belong to group II

( c ) What is the number of valence electrons in Y ?

Answer: – the element Y belongs to the group 15 hence the valance electrons
present in the Y element is 3.

(d) What is the valency of Y?

Answer: – the valency of the element Y is 3 it has 3 electrons in its outermost
shells which can be lost or gain.

23.

(a) What is a period in a periodic table ? How do atomic structures (electron arrangements) change in a period with increase in atomic numbers from left to right ?

Answer: – In the modern periodic table there are 7 horizontal rows which are
called as periods or series.

The change in the atomic structures with change in the period with increase in
atomic numbers when we move from left to right is that the atomic size
decreases due to increase in the electrons of the atom.

(b) How do the following change on going from left to right in a period of the periodic table?

(i) Chemical reactivity of elements
Answer: – when we move from left to right in a period of the periodic table the
chemical reactivity of elements decreases and then increases in some elements.

(ii) Nature of oxides of elements

Answer: – when we move from left to right in a period of the periodic table the
nature of oxides increases.

24.

a) How does the size of atoms (atomic size) generally vary in going from left to right in a period of the periodic table? Why does it vary this way?

Answer: – On moving left of right in the period of the periodic table atomic size
generally decreases. Atomic size decreases due to increase in the atomic number
of the elements. It is vary in that way because the atoic number increases that
means the number of proton and neutron increases which is attracted by the
nucleus and hence the atomic size reduces.

b) What happens to the metallic character of the elements as we move
from left to right in a period of the periodic table?
Answer: – as we move from left to right in a period of the periodic table the
metallic characters of the elements decreases.

25.

(a) Explain why :

(i) All the elements of a group have similar chemical properties.

Answer: – due to the same number of the valance electrons in the outermost
valance shell all the elements of a group posses same chemical properties.

(ii) All the elements of a period have different chemical properties.

Answer: – due to the different number of the valance electrons in the outermost
valance shell all the elements of a period posses different chemical properties.

(b) The atomic radii of three elements X, Y and Z of a period of the periodic
table are 186 pm ; 104 pm and 143 pm respectively. Giving a reason, arrange
these elements in the increasing order of atomic numbers in the period.

X = 186

Y = 104

Z = 143

The correct arrangement of these elements in their increasing order of atomic
numbers is – X<Y<Z

Because, the atomic size decrease as we move further along with period due to
increase in the atomic number of the elements.

26.

(a) How does the electropositive character of elements change on going
down in a group of the periodic table ?

Answer: – as we move down the group the electropositive characters of the
elements increases, because of adding new shells in it.

(b) State how the valency of elements varies

(i) in a group,

Answer: -The elements in the same group posses same valency.

(ii) in a period, of the periodic table.

Answer: – The elements in the period posses different valency. It goes on
increasing from 1 to 4 and then decreases to 0.

27. (a) What is the fundamental difference in the electronic configurations
between the group 1 and group 2 elements ?

The basic fundamental difference between the electronic configuration of the
group 1 and group 2 elements is that group 1 posses 1 valance electrons and
group 2 posses 2 valance electrons in its outermost shell.

(b) On the basis of electronic configuration, how will you identify :

(i) chemically similar elements ?

If the elements have same valance electrons in their outermost shell then they
will have same chemical properties because the chemical properties of the
element are dependant over the valance electrons.

(ii) the first element of a period ?

Answer: – if the element has only one electron in its outermost orbital then it is
said to be the first element of the period.

28.
(a) What is the usual number of valence electrons and valency of group 18
elements of the periodic table ?
Answer: – usually group 18 elements have 8 valance electrons in its outermost
shell and a valency is 0.

(b)What happens to the number of valence electrons in the atoms of elements as
we go down in a group of the periodic table ?
Answer: – the number of valence electrons in the atoms of elements will remain
same as we go down in a group of the periodic table.

29.
(a) What is the main characteristic of the last elements in the periods of the
periodic table ? What is the general name of such elements ?

Answer: – every period ends with the inert elements which have 0 valency. is the
characteristics of last element of period in the periodic table. The general names
of such elements are inert gases or Noble gases.

(b) What is the number of elements in : (a) 1st period, and (b) 3rd period, of the
modern periodic table ?

Answer: – the number of elements present in the modern periodic table are =

1) 1st period = 2

2) 3rd period = 8

30.
(a) How does the atomic size vary on going down from top to bottom in a group
of the periodic table ? Why does it vary this way ?

Answer: -As we move from top to bottom in a group of the periodic table the
atomic size increases due to addition of the new shells in the atoms of the
elements.

(b) Lithium, sodium and potassium are all metals that react with water to liberate
hydrogen gas. Is there any similarity in the atoms of these elements? Explain your answer.
Answer: – yes, there is a similarity between the Li, Na, and K, that is all of them
have only 1 valance electron in its outermost orbit, and due to that it is easily gets
react with the water and gives out hydrogen gas.

31.
a) How does the tendency to lose electrons change as we go down in group 1 of the periodic table? Why does it change this way?

Answer: – As we go down in group 1 of the periodic table the tendency to lose electrons will increase due to increase in the number of shell and the atomic radii of the atom. Atomic radius increases because the distance between the nucleus m and the electron of valance shell increases.

b) How does the tendency to gain electrons change as we go down in group 17 of the periodic table? Why does it change this way?

Answer: – as we go down in group 17 of the periodic table the tendency to gain
electrons decreases due to increasing size of an atom.

32.

(a) Why does the size of the atoms progressively become smaller when we move
from sodium (Na) to chlorine (Cl) in the third period of the periodic table?

Answer: – due to the increase in the atomic number the size of the atoms progressively becomes smaller when we move from sodium (Na) to chlorine (Cl)
in the third period of the periodic table.

(b) Helium and neon are unreactive gases. What, if anything, do their atoms have
in common?
Answer: – helium and neon belongs to the group 18 which have the completely
filled valance shell electrons. Hence these gases are known as inert gases or noble
gases, which are unreactive or less reactive.

33.

(a) In the modern Periodic Table, why does cobalt with higher atomic mass of
58.93 appear before nickel having lower atomic mass of 58.71 ?

Modern periodic table is based on the increasing atomic numbers of the elements
and not in the atomic masses hence in the modern periodic table cobalt having
atomic number 27 is placed before Nickel having atomic number 28.

(b)Why could no fixed position be given to hydrogen in Mendeleev’s periodic
table ?

Answer: – hydrogen posses the properties of both the alkali metals and halogens
group elements hence there is no fixed position given to the hydrogen in the
Mendeleev periodic table.

34.

a) What are the periods and groups in a periodic table ? Give two characteristics
of each.

In the periodic table there are 7 horizontal rows which are termed as periods.
1) Every element in the period has same number of shell.
2) Every element in the period is arranged as per the increasing order of the atomic number.

There are 18 vertical columns which are termed as groups.

Every element of the same group shows –

• 1) same electronic configuration
• 2) same chemical properties

b) In terms of electronic configurations, explain the variation in the size of the
atoms of the elements belonging to the same period and same group.

Answer: – Moving from left to right in the same period the size of the atom
decreases due to incease in the atomic numbers. While going down in the group
the atomic size of the atom increases due to increasing atomic shells.

C) Given alongside is a part of the periodic table. As we move vertically downward from Li to Fr :

(i) What happens to the size of atoms ?

Answer: – as we move vertically downward atomic size of the atoms will increases from Li to Fr.

(ii) What happens to their metallic character ?

Answer: – as we move vertically downward the metallic character of the atom will increases from Li to Fr.

d) Name two properties of elements whose magnitudes change when going from top to bottom in a group of the periodic table. In what manner do they change ?

Answer: – the two properties of the elements whose magnitude changes when going from top to bottom in a group of the periodic table are Francium and Lithium. Li has least metallic character whereas the Fr has most metallic characters. But both are present in the same group I of alkali metal elements.

e) Rewrite the following statement after correction, if necessary :

Groups have elements with consecutive atomic numbers.

Periods have elements with consecutive atomic numbers.

35.

(a) Explain why, the first period of the modern periodic table has only two elements whereas second period has eight elements

Answer: – there are 4 shells in each atom which are K,L,M,N which can accommodate only maximum number of 2, 8, 18, 32 electrons in it. The first period has outermost shell K which can accommodate only maximum number of 2 electrons in it. The second period has outermost shell L shell, hence it can accommodate a maximum 8 electron in it.

(b) Why do elements in the same group show similar properties but the elements in different groups show different properties?

Answer: – the elements in the same group have similar properties because they
show similar electronic configuration of the atoms, thus they have similar number
of valance electrons in their outermost shell. And the different groups shows
different properties because they have different electronic configuration and
different number of valance shell electrons.

(c) For each of the following triads, name the element with the characteristics
specified below :

Elements Least atomic and Chemically least radius reactive

(i) F, Cl, Br ………………. ……………….

Answer: – Elements Least atomic = F
Chemically least radius reactive = Br

(ii) Li, Na, K ………………. ……………….

Answer: – Elements Least atomic = Li
Chemically least radius reactive = Li

(d) State one reason for keeping fluorine and chlorine in the same group of the
periodic table.

Answer: – both the elements show similar chemical property because they have valency 1, hence they are placed in a same group of the periodic table.

(e) What are the merits of the modern periodic table of elements?

Answer: – following are the merits of the modern periodic table

• 1) Atomic numbers are the periodic functions of the elements – they are arranged as per the increasing atomic number of the elements in the periods. Periodicity of the elements is explained well.
• 2) Both the series lanthanides and actinides are placed independently
• 3) Isotopes of a certain elements are placed together according to their similar
atomic numbers.

36.

(a) What is a group in the periodic table ? In which part of a group would you separately expect the elements to have (i) the greatest metallic character (ii) the largest atomic size ?

Group– the vertical columns in the periodic table are known as groups
which are 18 in numbers. Both the greatest metallic characters and largest atomic
size are found at the lowest part of the groups. Because as we move down the
group the metallic characters and the atomic size increases.

(b) In what respects do the properties of group 1 elements differ from those of group 17 elements ? Explain with examples by taking one element from each group.

Answer: – group 1 have 1 valance electron in its outermost shell. these are
electropositive and alkali metals.

Group 17 have 7 electrons in its outermost shell, these are electronegative and
acids.

(c) From the standpoint of atomic structure, what determines which element will be the first and which the last in a period of the periodic table ?

Answer: – on the basis of the number of valance electrons present on the
outermost shell of the electrons the elements are determines to be first and the
last in the periodic table.

(d) Explain why, the properties of elements are repeated after 2, 8, 18 and 32 elements in the periodic table.

Answer: – there are four shells which are K, L, M, N and they can occupy only
limited number of the electrons in it; Which 2, 8, 18, 32 respectively. Thus the
properties of the elements are repeated after 2, 8, 18, 32 elements in the periodic
table.

(e) What are the advantages of the periodic table ?

Answer: – the periodic table is made for the easy study of the elements
properties. Due to the table all the periodic properties of each and every
elements is easily verified.

Multiple Choice Questions (MCQs)

37.) Which of the following statements is not a correct statement about the trends
when going from left to right across the periods of the periodic table ?

(a) The elements become less metallic in nature.

(b) The number of valence electrons increases.

(c) The atoms lose their electrons more easily

(d) The oxides become more acidic.

Answer: -(c) The atoms lose their electrons more easily

Explanation – moving from left to right in the periods of modern periodic table the atoms does not lose electrons easily.

38. The electronic configuration of the atom of an element X is 2, 8, 4. In modern
periodic table, the element X is placed in :

(a) 2nd group (b) 4th group (c) 14th group (d) 8th group

Explanation – it has 4 electrons in its outermost shell, according to the electronic
configuration that element should be placed in 14th group.

39. The atomic number of an element is 20. In modern periodic table, this element is placed in :

(a) 2nd period (b) 4th period (c) 3rd period (d) 1st period

Explanation – according to the atomic number calcium 20 will be placed in the 4th
period.

40. Five elements A, B, C , D and E have atomic numbers of 2, 3, 7, 10 and 18
respectively. The elements which belong to the same period of the periodic table
are :

(a) A, B, C (b) B, C, D (c) A, D, E (d) B,D,E

Explanation – all these three have same number of valance shell thus they are
present in same period.

41. The elements A, B, C, D and E have atomic numbers 9, 11, 17, 12 and 13
respectively. The pair of elements which belongs to the same group of the
periodic table is :

(a) A and B (b) B and D (c) A and C (d) D and E

Answer: – (c) A and C

Explanation – they both have same number of valance electrons in its outermost
shell hence they are present in same group which is 17.

42. Which of the following element would lose an electron easily ?

(a) Mg (b) Na (c) K (d) Ca

Explanation – potassium would lose an electron easily because it has only one
atom in its outermost valance shell electrons.

43.) Which of the following element does not lose an electron easily

(a) Na (b) F (c) Mg (d) Al

Explanation – fluorine element does not lose an electron easily because it has 7
electrons in its outermost shell.

44. Where would you locate the element with electronic configuration 2, 8 in the
modern periodic table ?

(a) group 8 (b) group 2 (c) group 18 (d) group 10

Explanation – group 18 has the maximum 8 valance shell electrons in its
outermost shell hence it is group 18.

45. An element which is an essential constituent of all organic compounds
belongs to following group of modern Periodic table:

(a) group 4 (b) group 14 (c) group 15 (d) group 16

Explanation – the essential constituent of the organic compound is Carbon preset
in the group 14 of periodic table.

46.) Which of the following is the valence shell for the elements of second period
of the modern periodic table?

(a) M shell (b) K shell (c) L shell (d) N shell

Explanation – second period of the modern periodic table has valance shell L.

47. The element which has the maximum number of valence electrons is:

(a) Na (b) P (c) Si (d) Al

Explanation – the maximum valance shell electrons among the given options are
near the phosphorous which is 5.

48. The correct increasing order of the atomic radii of the elements oxygen,
fluorine and nitrogen is :\

(a) O, F, N (b) N, F, O (c) O, N, F (d) F, O, N

Explanation – atomic radius decreases from left to right in period and increases
from down to bottom in the group. Hence the correct increasing order is F,O,N.

49. The atomic numbers of the elements Na, Mg, K and Ca are 11, 12, 19 and 20
respectively. The element having the largest atomic radius is :

(a) Mg (b) Na (c) K (d) Ca

Explanation – potassium has the largest atomic radius among the given options.

50. Which of the following are the correct characteristics of isotopes of an
element ?

(i) same atomic mass (ii) same atomic number
(iii) same physical properties (iv) same chemical properties

(a) (i), (iii) and (iv) (b) (ii), (iii) and (iv) (c) (ii) and (iii) (d) (ii) and (iv)\

Explanation – isotopes are the elements which posses same atomic numbers and
the same chemical properties. But different atomic mass numbers and different
physical properties.

51. The correct formula of the oxide of Eka-aluminium element predicted by
Mendeleev was :

(a) EaO3 (b) Ea3O2 (c) Ea2O3 (d) EaO

Explanation – these is the correct formula of the eka- aluminum predicted by the
Mendeleev.

52. The element which can form an acidic oxide should be the one whose atomic
number is :

(a) 6 (b) 16 (c)12 (d) 19

Explanation – atomic number 16- the name of the element is sulphur (S) is the element which form the acidic oxide. For example sulphur dioxide.

53. The element which forms a basic oxide has the atomic number of :

(a) 18 (b) 17 (c) 14 (d) 19

Explanation – atomic number 19 the name of the element is potassium (K) is the
element which form the basic oxide.

54. Which one of the following does not increase while moving down the group of
the periodic table ?

(a) atomic radius (b) metallic character (c) valence electrons (d) shells in the
atoms

Explanation – atomic radius, metallic characters and the shells in the atoms all
these increases while moving down the group of periodic table but valence
electrons does not increases.

55. On moving from left to right in a period of the periodic table, the atomic number of elements increases. What happens to the size of atoms of elements on
moving from left to right in a period ?
(a) increases

(b) decreases

(c) remains the same

(d) first increases then decreases

Explanation – the size of the atom decreases when we move from left to right in
the periods of the periodic table.

56. Which of the following set of elements is written correctly in the order of their
increasing metallic character?

(a) Mg, Al, Si (b) C, O, N (c) Na, Li, K (d) Be, Mg, Ca

Explanation – these elements are present in the group 2. And these are correctly
written in their increasing order of the metallic characters.

### This Chapter Video Explanation

Part 2 Video – https://youtu.be/jpocO0GR10w

Part 3 Video – https://youtu.be/sG34SN72d0A

Part 4 Video – https://youtu.be/xG_H8ELkEGU

For more Chapter solution, click below

Updated: November 16, 2022 — 5:04 pm