Lakhmir Singh Manjit Kaur Class 9 Chemistry 4th Chapter “Structure of Atom” solution
Lakhmir Singh and Manjit Kaur Chemistry solution: “Structure of Atom” Chapter 4. Here you get easy solutions of Lakhmir Singh and Manjit Kaur Chemistry solution Chapter 4 . Here we have given Chapter 4 all solution of Class 9. Its help you to complete your homework.
- Board – CBSE
- Text Book – Chemistry
- Class – 9
- Chapter – 04
Lakhmir Singh Manjit Kaur Class 9 Chemistry 4th Chapter Solution (First Part)
1> Which subatomic particle is not present in an ordinary hydrogen atom?
Ans. Hydrogen does not consists of neutron as a subatomic particle and that is why it’s mass is only 1.
2. Name the scientist who described the arrangement of electrons in an atom.
Ans. J.J Thomson described the arrangement of electrons in an atom.
3. What is the maximum number of electrons which can be accommodated in the K-shell?
Ans. K-shell can accommodate maximum of 2-electrons.
4. What is the maximum number of electrons which can be accommodated in L-shell?
Ans. L-shell can accommodate maximum of 8-electrons.
5. What is the maximum number of electrons M-shell can accommodate?
Ans. M-shell can accommodate maximum of 18 electrons.
6. What is the maximum number of electrons that can go in N-shell?
Ans. N-shell can accommodate maximum of 32 electrons.
7. What is the maximum number of electrons which can be accommodated in the
a) Innermost shell of an atom
Ans. Innermost shell of an atom can have maximum of 2-electrons.
b) Outermost shell of an atom
Ans. Outermost shell of an atom can have maximum of 8-electrons.
9. Name the negatively charged particle present in the atoms of all elements.
Ans. Electron is the negatively charged particle present in all atoms.
10. Name the scientist who discovered electron.
Ans. J.J Thomson discovered electron.
11.What is the usual symbol for
a) an electron
b) a proton
C) a neutron
12.State whether the following statements are true or false
a) Thomson proposed that the nucleus of an atom contains protons and neutrons.
Ans. FALSE. Thomson proposed a model where numbers of electrons and
protons were equal.
b)The cathode rays obtained from all the gases consists of negatively charged particles called electrons.
Ans. TRUE. Cathode rays are formed due to ejection of electrons which are
c)The anode rays obtained from all the gases consist of positively charged
particles called protons.
Ans. FALSE. Anode rays are due to positive charge and not all gases can
dissociate to give a cation.
13> Name the central part of an atom where protons and neutrons are held together.
Ans. The central part of any atom which consists of neutron and proton held together is called nucleus.
14. What are the various letters used by Bohr to represent the electron shells in an atom.
Ans. Bohr used K,L,M and N for the 1st,2nd,3rd and 4th shell respectively.
15> Name the particles which actually determine the mass of an atom.
Ans. Neutron and proton added up gives the mass of an atom due to significant values of mass numbers.
16.Name the positively charged particle present in the atoms of all the elements.
Ans. Proton is the positively charged particle present in atoms of all elements.
17> What is the electronic configuration of a hydrogen atom .
Ans. Electronic configuration of H : (K)1.
18> How many times a proton is heavier than an electron?
Ans. Proton is heavier than electron. Proton= 1836 times of electron.
19> Name the gas which produces anode rays consisting of protons in the discharge tube experiment.
Ans. Hydrogen gas produces anode rays consisting of protons in the discharge tube experiment.
20> Which part of an atom was discovered by Rutherford’s alpha particle scattering experiment?
Ans. Nucleus was discovered by Rutherford’s alpha particle scattering experiment.
21> What his the positive charge on the nucleus of an atom due to?
Ans . The positive charge is due to protons present in the nucleus which are positively charged
SHORT ANSWER TYPE QUESTIONS
29> What is the absolute mass and charge on electron?
Ans. Absolute mass of electron: 9.10938356 × 10-31 kg
30.Give the evidence for the existence of nucleus in an atom.
Ans. Rutherford’s alpha particle scattering experiment gave the evidence of existence of nucleus. The positively charged alpha particles were deviating from the center of the atom which explained the existence of a heavy positively charged bulk which was later termed as “nucleus”.
31. What important information is furnished about the nucleus of an atom by the alpha particle scattering experiment of Rutherford ?
Ans. These were the most important information
1) Atom consists of a heavy central part called as nucleus where mass of the whole atom is concentrated
2) Atom has a lot of empty space present in it as lots of particles were going undeviated in a straight line.
32.How was it shown that an atom has a lot of space within it?
Ans. Rutherford’s gold foil experiment explained that when beam of alpha particles is passed, majority of them go straight and does not deviate which explains that there must be empty space for that much particles to pass.
33.Why is an atom neutral inspite of the presence of charged particles in it?
Ans. Atom is a neutral species due to the presence of equal number of electrons(-vely charged particles) and protons (positively charged particles). The third subatomic particle called as neutron has no charge on itself and so does not contributes to the overall charge of the atom.
34> a) Which of the nuclear particles is present in the same fixed number in the atoms of any particle element.
Ans. Proton is present in the same fixed number in any particle element which explains why each element has a different atomic number.
b) What do we call this number which is characteristic of a particular element?
Ans . Atomic number is the characteristic of a particular element which is different for different elements. Therefore no two elements can have same atomic number.
35. What is a proton? State its relative mass and charge.
Ans. Proton is a subatomic particle which is positively charged and represents the atomic number of any element.
Relative mass= 1
Charge=1.67262 × 10−27kg
36. What is absolute mass and charge of a proton?
Ans. Absolute mass: 1.67× 10−27kg .
Charge = 1.6×10−19C .
38.State two observations which show that atom is not indivisible.
Ans. Two major experiments showed that atom is not indivisible:
i) J.J Thomsons cathode ray experiment: Cathode rays in the discharge tube shows the presence of electrons.
ii) E .Goldstein’s experiment: faint red glow in the gas discharge tube shows the presence of protons.
39.All the gases form cathode rays and anode rays when electricity is passed through them
i) What does the formation of cathode rays tell us about the atoms?
Ans: Cathode rays shows the presence of electrons in atom.
i)What does the formation of anode rays tell us about the atoms?
Ans. Anode rays shows the presence of protons in atom.
40.What do you understand by the term “electronic configuration” of an element? Write down the electronic configuration of oxygen (At.No. 8).
Ans. The distribution of electrons in shells K,L,M &N respectively is called as electronic configuration.
O (8)= K(2), L(6)
41.An element has an atomic number 12. How many electrons will be present in the K,L and M energy shells of its atom.
Ans. The element with atomic number 12 is Magnesium. Mg(12):K(2),L(8) , M(2)
42. a) What is the nucleus of an atom and what is the nature of charge on it?
Ans. Nucleus of an atom is the central part which consists of neutrons and protons which contributes to the mass of an atom. Nucleus is positively charged as it contains protons and neutrons where protons contributes to the positive charge and neutrons are neutral.
43. Name the particles used by Rutherford in his experiment on the discovery of nucleus. Also state the charge on these particles.
Ans. The particle used in Rutherford’s experiment was alpha particle having a charge of +2.
44. An element has atomic number 13 and an atomic mass of 27.
a)How many electrons are there in each atom of the element.
Ans. 13 electrons are present. The element is Aluminium.
b)How are these electrons distributed in the various energy levels.
Ans. Electronic configuration: K(2),L(8),M(3)
45.Write the distribution of electrons in an atom of element whose atomic number is 18.What is the special about the outermost electron shell (or valence shell) of the atom of this element?
Ans. Element with atomic number 18 is Argon with electronic configuration K(2),L(10),M(8)
The outermost shell consists of 8- electrons which makes the atom very stable and therefore Argon belongs to noble gases.
46.What is a neutron? State the relative mass and charge.
Ans. Neutron is a subatomic particle which is present inside the nucleus.
Relative mass = 1
Charge = 0
47.Compare an electron, a proton and a neutron in respect of their relative masses and charges.
48.What is a proton? How does it differ from a neutron?
Ans. Proton is a positively charged subatomic particle which is inside the nucleus. Proton and neutron both have the same masses but proton is positively charged and neutron is neutral.
49. Compare an electron and a proton in respect of mass and charge.
50. Compare a proton and neutron in respect to mass and charge.
52.State the location of electron, proton and neutron in an atom.
Ans. Electron : Revolving around the nucleus in circular shells. Proton: Inside the nucleus. Neutron: Inside the nucleus with proton.
53.Fill in the following blanks:
|Atomic Number||Mass Number||Protons||Neutrons||Electrons||Symbols|
54. Fill in the following blanks in respect of an atom of an element.
|No. of Protons||No. of Neutrons||Mass Number||Atomic number||No. of Electrons||Symbol|
LONG ANSWER TYPE QUESTIONS
55> a) What are cathode rays? What is the nature of charge on cathode rays? Ans. Cathode rays are a beam of electrons which emerge from the cathode of a high vacuum discharge tube. As they emerge from cathode, the charge on cathode rays is negative.
b) Explain how ,cathode rays are formed from the gas taken in discharge tube
Ans. The gas taken in the discharge tube gets ionized by the small amount of electricity passed through it and a green glow on the wall was observed which was due to the rays coming from the cathode, and because the rays were coming from cathode they were named as cathode rays.
c) What conclusion is obtained from the fact that all the gases form cathode rays?
Ans. All the gases can form cathode rays explains the fact that every gaseous atom consists of electrons as a subatomic particle which can ionize.
56. a) Describe Thomson’s model of the atom. Which subatomic particle was not present in Thomson’s model of the atom?
Ans. Thomson’s atomic model was based on the fact that an atom is a neutral species. Following are the postulates of Thomson’s atomic model:
i) The atom is a positively charged sphere.
ii) Electrons are embedded in it in the same quantity as the protons which makes the atom neutral. Thomson’s atomic model had no inclusion of neutron as a subatomic particle.
b) The mass of number of an element is 18.It contains 7 electrons. What is the number of protons and neutrons in it? What is the atomic number of the element?
Ans. No. of electrons = no. of protons= 7
Mass no. = no. of protons + no. of neutrons= 18
18 = 7 + x , therefore x= 11 (neutrons).
Atomic number = 7 ( Nitrogen)
57. a) Describe the Rutherford’s model of an atom . State one drawback of Rutherford’s model of an atom.
Ans. Rutherford’s atomic model has the following postulates :
i) The center of the nucleus consists of positively charged heavy part called as nucleus where the mass of an atom is concentrated.
ii) Atom consists of a lot of empty space which explains why majority of the beam passed without deviation.
iii) One in 12000 rays was reflecting back which explained the positive nature of nucleus.
Rutherford said that electrons were revolving around the nucleus in a circular manner, but if electrons are revolving then at some point it should fall inside the nucleus due to attraction between negatively charged electron and positively charged nucleus which if happens then there will be collapsing of atom which is not a possible case.
b) The mass number of an element is 23. It contains 11 electrons. What is the
number of protons and neutrons in it? What is the atomic number of the
Ans. No. of electrons = no. of protons = atomic number = 11 (Sodium)
Mass no.= no. of protons + no. of neutrons
23 = 11 + x , therefore x=12 ( neutrons)
58. a)Describe Bohr’s model of the atom. How did Neils Bohr explain the stability of an atom.
Ans. Following are the postulates of Bohr’s atomic model:
i) Electrons are present only in certain orbits which are stationary and stable in
ii) He called the energy levels as discrete which means every orbit has its
specific energy and levels.
iii) He named the energy levels as K,L,M and N having orbit numbers as 1,2,3
and 4 respectively.
iv) Electron capacity of each orbit is given by the formula 2n^2.
K-shell (n=1) = 2×1^2= 2
L-shell (n=2) = 2×2^2=8
M-shell (n=3) = 2×3^2=18
N-shell (n=4)=2×4^2 = 3
b) An element has an atomic number of 11 and its mass number is 23. What is the arrangement of electrons in the shells? State nuclear composition of an atom of the element.
Ans. K-shell = 2
#Nuclear composition of sodium
59. What is meant by
a) i)Atomic number: It is the number of protons present in an atom.
ii)Mass number: It is the number of protons and number of neutrons combined.
|Element||Atomic Number||Mass Number|
|Carbon||6||6+6 = 12|
b) What is the relation between the atomic number and the mass number of an element?
Ans. Atomic number is the number of protons and number of neutrons combined.
A = Z + n where; A = atomic mass
Z= atomic number
n= number of neutrons
c) If an element M has mass number 24 and atomic number 12, how many
neutrons does its atom contain?
Ans. Mass number = atomic number + number of neutrons
24 = 12 + x
Therefore x = 12 ( neutrons)
Lakhmir Singh Manjit Kaur Class 9 Chemistry 4th Chapter Solution (Second Part)
VERY SHORT ANSWER TYPE QUESTIONS
Ans. Nitrogen has 7 electrons; N7: 2,5 therefore valence electrons (8-5) = 3.
Ans. The general name for elements having 8 electrons in the valence shell is noble gas elements as they are highly stable and do not react easily.
Ans. Helium has less than 8 electrons in the last shell as helium has only 2-electrons in its atom.
Ans. Iodine isotope I-131 is used in the treatment of hyperthyroidism.
Ans. Radioactive isotope used in nuclear power plant is Uranium-238.
Ans. Yttrium-90 is used in treatment of cancer.
Ans. Radioactive isotope used in determination of activity of thyroid glands is I-131.
Ans. Radioactive isotope used industry is Co-60 for finding out the faults in pipelines and welds.
Ans. TRUE. Radioactive isotope of I-127 is used as it is stable in nature.
Ans. These type of atoms which have same number of protons and same number of electrons but different number of neutrons are called as isotopes as change in number of neutrons changes the mass number.
Ans. Both are isotopes of each other as they have same number of protons but different mass numbers.
i) 11-protons + 11- neutrons = 22 amu.
ii) 11-protons + 12- neutrons = 23 amu.
Ans. These both are isotopes of each other as they have same atomic number but different atomic masses.
Ans. The isotopes which have unstable nuclei and emit various types of radiations are called as radioactive isotopes. For example: U-235 isotope is used in nuclear power plants to generate electricity which gives out radiations.
|Number of protons||Numbers of neutrons||Mass number||Atomic number||Number of electrons||Valency|
15] Complete the following statements:
(a) Magnesium has 2 valence electrons in the outermost(M) shell.
(b) The valancy of nitrogen in N2 molecule is 3.
(c) Isotopes have different mass numbers because their nuclei contain different number of neutrons .
(d) Some boron atoms have mass number 10 and some have mass number 11. These boron atoms with different mass numbers are called isotopes .
SHORT ANSWER TYPE QUESTIONS
16] The nucleus of an atom has 5 protons and 6 neutrons. What would be the
(a) atomic number
Ans. Atomic number will be 5 because protons give the atomic number.
(b) mass number
Ans: Mass number will 11 (mass of protons + mass of neutrons).
(c) the number of electrons
Ans. Number of electrons will be 5 in the neutral atom to balance the number of protons.
(d) the number of valence electrons, per atom of this element?
Ans: Valency will depend on the electronic configuration therefore; E5: 2,3. Valency= 3.
Ans. Element with atomic number 17 is chlorine. Electric configuration Cl17: 2,8,7.
(a) Write down the electronic configuration of X.
Ans. Element with atomic number 16 is sulphur. S16: 2,8,6.
(b) What will be the valancy of X?
Ans. The valency of S16 : 8-6=2.
Ans. The valencies shown by elements will be :
i) A2: 2
ii) B4: 2,2
iii) C8: 2,6
iv) D10: 2,8
v) E13: 2,8,3
Ans. Uranium-235 is used as a nuclear fuel in nuclear power plants.
Ans. Cobalt-60 is used to find the welds and defaults in pipelines and industries.
Ans. Isotopes are atoms of same elements with same atomic number and different atomic masses. H13 and H23 do not have the same atomic numbers and therefore they are not isotopes but isobars having same atomic masses.
Ans. Atomic masses are different in the case of isotopes due to the different number of neutrons present in atoms.
Ans. Chemical properties are a function of number of electrons which does not changes in the case of isotopes. Isotopes are atoms of same element with different atomic masses but with same number of electrons.
For ex : Cl1735 and Cl1737 have the same number of e– and therefore will have the same chemical property but different atomic mass.
Ans. Isotopes of the same element has different atomic masses which contributes to the physical properties such as density, M.P/B.P etc. thus different atomic masses will give different physical properties.
Ans. Due to the existence of isotopes in nature the average atomic mass of most the elements comes out to be in fraction.
For ex: Cl35 and Cl37 are two isotopes of chlorine and the mass of Cl is taken as 35.5 amu which is average of both with respect to the abundance in nature.
Ans.i) Protium, Deuterium and Tritium are isotopes of each other having same atomic number but different atomic masses. (H11 , H12 and H13).
ii) Argon and Calcium are isobars of each others as they have different atomic numbers but same atomic masses. (Ar1840 and Ca2040 ).
(i) These isotopes have almost identical chemical properties.
Ans. Due to the same number of electrons present in the atom the chemical properties will be similar.
(ii) They are electrically neutral.
Ans. The number of electrons and protons remain the same and the is why the atom is neutral. The difference in mass number is due to change in number of neutrons which is electrically neutral and does not contribute to the charge of the atom.
Ans. Average atomic mass of neon = (mass of 1st isotope/100) x abundance +
(mass of second isotope /100) x abundance
=(20/100) x 90 + (22/100) x 10
Ans. Isobars are atoms of different elements with different atomic numbers and same atomic masses.
For ex: Argon and Calcium are isobars of each others as they have same mass numbers but different atomic numbers. Ar1840 and Ca2040 are isobars of each other.
Ans. i) H11 : 1 , electrons = 1 , protons = 1 , neutrons = 0
ii) H12 : 1 , electrons = 1 , protons = 1 , neutrons = 1
iii) H13 : 1 , electrons = 1 , protons = 1 , neutrons = 2.
Ans. Element with atomic number 7 is nitrogen
Electronic configuration N7 : 1,5. Valency: (8-5)= 3.
LONG ANSWER TYPE QUESTIONS
Ans. Valence electrons are the number of electrons participating to form a bond. Valence electrons are present in outermost shell of an atom also called as valence shell.
Ans. Element with atomic 13 is Aluminium. Valence electrons Al13 : (2,8,3 ) = 3.
Ans. Isotopes are atoms of same element having same atomic number but different atomic masses.
Example: C612 and C614 are isotopes of each other with same atomic number but different atomic masses.
Ans. Similarity: They have same atomic numbers.
Difference: They have different atomic masses due to different number of neutrons.
Ans: Cl1735: protons = 17 , electrons = 17 , neutrons = 18.
Cl1737 : protons = 17 , electrons = 17 , neutrons = 20.
Ans. Atoms of same element whose nuclie are unstable and radiate large amount of energies in the form of alpha ,beta or gamma particles. Ex: U-235 and C0-60.
(b) two uses of radioactive isotopes.
ii) U-235 is used as a nuclear fuel in nuclear power plants to produce electricity.
Ans. Average atomic mass : (mass of the 1st isotope/100) x abundance +( mass of 2nd isotope / 100) x abundance .
35.5 amu : (35/100) X x1
Ans. Valency : It is the combining capacity of an element which is generally the number of electrons present in the outermost shell. Element with atomic number 14 is silicon and its valency will be
Si14 : 2,8,4 therefore the valency will be 4.
Ans. Valency is the combining capacity of any element which is the number of electrons gained or lost to form bond whereas the number of electrons present in the outermost shell is called as the valence electrons. Both can be same and can be different depending upon the number of electrons.
For ex. Cl17: 2,8,7 ; valency = 8-7=1
C6 : 2,4 ; valency = 4
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