Selina Concise Class 8 Chemistry Solution Chapter No. 4- ‘Atomic Structure’ For ICSE Board Students.
1)
a) Dalton said that ……………could not be divided.
Answer : Atom
Explanation: Dalton said that matter made up of atoms which is very tiny and small particles , it cannot be break or divided further.
b) An ion which has a positive charge is called a ………………
Answer : Cation
Explanation: when electrons are transfer and charge occur on the atom called ion cation are form due to the loss electrons.
c) The outermost shell of the atom is known as …………………
Answer : Valence shell
Explanation: The outermost shell of the atom consist 1 to 8 electron these electrons present in the last shell of the atom called valence electrons that’s why the outermost shell of the atom known as valence shell.
d) The …………….of an atom is very hard and dense.
Answer : Nucleus
Explanation: Nucleus is very hard and dense it is present in the centre of atoms and electrons are moving around it , the mass of electrons are negligible ,so inside the nucleus neutrons and protons are present , more 99 % mass consists through the nucleus of atom.
e) Neutrons are ………………particles having mass equal to that of protons.
Answer : neutral
Explanation: Neutrons are sub Atomic particles present inside the nucleus of atom , it has neither negative nor positive charge . The weight of neutrons are slightly more than proton.
f) Isotopes are the atom of …………….. element having the ………………atomic number but ………………mass number .
Answer : Same , Same , Different
Explanation: Isotope are the same elements has same atomic number but different mass number because it has same number of protons but different number of neutrons.
2)
a)
Answer : False
(Atom has negative charge electrons , and positive charge protons that’s as whole atoms are neutral.)
b)
Answer : False
(The maximum number electrons in shell determine by the formula 2n2 so in the first shell n = number of electrons one 2n2 = 2×12 = 2 , so the first shell has only two electrons)
c)
Answer : True
(Nucleus is present in the centre inside the atom)
3)
a)
Answer : Electrons
(The subatomic particles with negative charge and negligible mass are known as electrons )
b)
Answer : Nucleons
(The number of protons and neutrons present in the nucleus are known as Nucleons )
c)
Answer : Valence shell
(The electrons present in the outermost shell are known as valence shell)
d)
Answer : Electronic Configuration
( Arrangements of electrons in shell of the atoms are called electronic configuration ).
e)
Answer : Atomic number
(The number of protons present in the nucleus of an atom is known as atomic number )
f)
Answer: Mass number
(The sum of the number of protons and neutrons present of an atoms are known as mass number)
g)
Answer : Isotope
(Atom of same elements with same atomic number but different mass number are known as isotope )
h)
Answer : Atom
(The smallest unit of an element which takes part in a chemical reaction are called atom)
4)
a) The outermost shell of an atom is known as
Answer : (iv) Valence shell
(The last shell of electrons known as valence electrons so the last shell or outermost shell is also known as valence shell).
b) The number of valence electrons present in magnesium is
Answer : (i) two
(Magnesium Electronic Configuration is 2,8,2 So the outermost shell have 2 Electrons hence the Valency of magnesium is two).
c) The subatomic particle with positive charge is
Answer : (i) proton
(The subatomic particles present inside the atom Electrons has negative charge, protons and neutrons present inside the nucleus of atom, protons have positive charge and neutrons are neutral).
d) If the Atomic number of an atom is 17 and mass number is 35 , then number of neutrons will be
Answer : (iii) 18
(The mass number is sum of number of protons and neutrons So proton has 17 and mass number is 35 then 17-35 = 18).
e) The number of electrons in an atom is equal to the number of
Answer : (i) protons in a neutral atom
(When the number of protons and electrons are equal in the atom are known as neutral atom).
f) The sum of number of protons and number of neutrons present in the nucleus of an atom is called it’s
Answer : (i) Mass number
(sum of the protons and neutrons present in the nucleus of atom are known as mass number which is represented by A).
5)
Answer: Three fundamental particles of an atom are electrons , protons and neutrons.
a) Electron : symbol of electrons is e–one unit negative charge 1.602×10-9 coulomb .
b) Proton :. Symbol of proton is P+ , positive charge of protons same 1.602×10-19 Coulomb is same to electrons charge unit.
c) Neutron : Symbol of neutron is (n) which has no charge or zero charge why it is neutral.
6)
a)
Answer : Atomic number :
The number of protons or electrons present in a element or atom is called as Atomic number . It is represented by Z.
b)
Answer : Mass number
In the elements total or sum of number of protons and number of neutrons inside the nucleus of atom is called as mass number . It is represented by A
c)
Answer : Nucleons :
The sub atomic particles like protons and neutrons are considered inside the nucleus are known as Nucleons.
d)
Answer : Valence shell :
Last shell or orbit of the elements or atom is called as valence shell.
7)
Answer : Silent features of Dalton’s atomic theory :
- Matter is made up small or tiny particles that is atom.
- Atom are small particles and not divided in further , and not destroyed .
- Atom take a part in chemical reaction of matter .
- Small atom combine together to form a molecules.
- All atom have same or identical properties of that elements.
- Atom of elements combine to another atom or elements in a fixed proportion.
8)
a)
Answer : Two main features of Rutherford atomic model are following.
- Atom has nucleus in a centre of atom.
- The nucleus has positive charged.
- Electron rotate around the nucleus on orbit.
- Size of nucleus small as compared to atom .
b) State its one drawback?
Answer : Drawback of Rutherford atomic theory :
- In a Rutherford atomic theory, Rutherford can’t explain stability of atoms.
9)
Answer : Rutherford observation are following :
- When Rutherford bombarded or blast thin gold foil with alpha particles passed or go straight by gold foil .
- But in a small point some alpha particles are refract or scattered .
- From this experiment Rutherford get the point that in the centre of atom has heavy mass positive charge particles are present . After that it has give the name Nucleus present in the centre of atom.
10)
Answer :
a) Carbon ( 6C12)
- Mass number of carbon is twelve (12).
- Atomic number of carbon is six (6).
- Number of neutron in carbon atom are six (6).
- Electronic configuration of carbon is 2,4
- Atomic diagram of carbon is following.
b) Oxygen ( 8O16)
- Mass number of oxygen is sixteen (16).
- Atomic number of oxygen is eight (8).
- Number of neutron in oxygen atom are eight(8).
- Electronic configuration of carbon is 2,6.
- Atomic diagram of oxygen is following.
c) Neon ( 10Ne20)
- Mass number of neon is twenty (20).
- Atomic number of carbon is ten (10).
- Number of neutron in neon atom are ten (10).
- Electronic configuration of oxygen is 2,8.
- Atomic diagram of neon is following.
d) Fluorine ( 9F19)
- Mass number of fluorine is nineteenth (10).
- Atomic number of carbon is nine (9).
- Number of neutron in fluorine atom are ten (10).
- Electronic configuration of Fluorine 2,7.
- Atomic diagram of fluorine is following.
e) Aluminium ( 13Al27)
- Mass number of aluminium is twenty seven (27).
- Atomic number of aluminium is thirteen (13).
- Number of neutron in aluminium atom are fourteen (14).
- Electronic configuration of aluminium is 2,8,3.
- Atomic diagram of aluminium is following.
f) Chlorine ( 17Cl35)
- Mass number of chlorine is thirty five (35).
- Atomic number of chlorine is seventeen (17).
- Number of neutron in chlorine atom are eighteen (18).
- Electronic configuration of chlorine is 2,8,7
- Atomic diagram of chlorine is following.
11)
Answer: variable Valency :
The elements have different capacities or Valency or more than one Valency are called variable Valency .
Name the elements which has variable Valency are following.
1) Copper (Cu) :
Copper has two one (1) and two (2) ,Valency when Cu is chemically combine with oxygen , to form cuprous oxide Cu2O , and cupric oxide CuO.
2) Iron (Fe) :
Iron has two Valency +2 and +3 , when iron is combine with oxygen to form ferrous oxide which has the valence +2 and ferric oxide has the valence + 3
12)
Answer : The atomic number and the mass number of sodium 11 and 23 respectively . This statement is conveyed that in sodium elements proton and electrons are same that is 11 Atomic number of sodium and the total sum of protons and neutrons are 23 mass number of sodium .
13.)
a)
Answer : Nitrogen
Electronic configuration : 2,5
b)
Answer : Neon
Electronic configuration : 2,8
14)
Answer :The energy levels or shell has the capacity of maximum number of electrons are determined by the formula 2n2
n = number of shell or energy levels.
The energy levels or shell on that electrons revolve around the nucleus which are following .
1) K Shell ;
K Shell is near to the nucleus have the low energy and minimum capacity of electrons that only two electrons can fill in the shell .
- In k shell has the capacity of two electrons
- Formula = 2n2
- n = number of shell or energy levels .
- K shell = 2 ×12
= 2
- Example : Helium
- Electronic configuration helium 2.
2) L Shell :
L Shell is next after the K Shell which has the more energy as compared toK shell , the capacity of L shell are 8 electrons.
- Formula = 2n2
- n = number of energy levels or shell
- L Shell = 2× 22
- =2×4 = 8
- Example : Neon
- Electronic configuration Neon 2,8 .
3) M Shell :
M Shell is far from nucleus which has the more energy as compared to L shell , the capacity of M shell are 18 electrons.
- Formula = 2n2
- n = number of energy levels or shell
- L Shell = 2× 32
- =2×9 = 18
- Example : Argon
- Electronic configuration of Argon , 2,8,8
4) N Shell :
N Shell is very far from nucleus which has the more energy as compared to M shell , the capacity of N shell are 32 electrons.
- Formula = 2n2
- n = number of energy levels or shell
- L Shell = 2× 42
- =2×16 = 32
- Example : Potassium (K)
- Electronic configuration of potassium , 2,8,8,1
15)
Answer :. Beryllium is the element which is made up of 4 protons, 5 neutrons and 4 electrons . Atomic number of this element have 4 and mass number is 9.
16)
a)
Answer : Atom is made up of two main parts which is following.
1) Atom has nucleus in the centre which has sub Atomic particles protons and neutrons.
2) Atom has orbit or shell where electrons revolve around the nucleus .
b)
Answer : Nucleus of an atom situated in the centre of an atom.
c)
Answer : The path where electrons revolve or rotate around the nucleus such path is known as shell or orbit.
17)
Answer : Those elements or atom have same number of protons and electrons that is atomic number and Nucleus has different number of neutrons that is mass number are called isotope .
Isotope contradict Dalton’s atomic theory:
Dalton’s said that all atom or elements are same but it is false ,
All atom or elements are not same . Isotope is that atom in which same atomic number that is electrons and protons are samebut different neutrons present in the nucleus that is mass number is different.
18.)
Answer :
Elements | Symbol | Number of protons | Number of neutrons | Number of electrons |
Fluorine | 9F19 | 9 | 10 | 9 |
Aluminium | 13Al27 | 13 | 14 | 13 |
Potassium | 19K39 | 19 | 20 | 19 |