NCERT Solutions for Class 8 Science Chapter 4 - Structure of the Atom

Q1. What are canal rays?
 Ans. Canal rays are positively charged radiations which led to the discovery of positively charged sub-atomic particle called proton.
 

Q2. If an atom contains one electron and one proton, will it carry any charge or not?
 Ans. The atom will be electrically neutral as one − ve charge balances one + ve charge.
 

Q3. On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole.
 Ans. According to Thomson’s model of an atom
(i) An atom consists of a positively charged sphere and the electrons are embedded in it.
(ii) The negative and positive charges are equal in magnitude. So the atom is electrically neutral.
 

Q4. On the basis of Rutherford’s model of an atom, which sub-atomic particle is present in the nucleus of an atom?
 Ans. As per Rutherford’s model of an atom, the protons which are positively charged are present in the nucleus of an atom.
 

Q5. Draw a sketch of Bohr’s model of an atom with three shells.
 Ans.

Q6. What do you think would be the observation if the α-particle scattering experiment is carried out using a foil of a metal other than gold?
 Ans. On using any metal foil, the observations of the α-particle scattering experiment would remain the same as all atoms would have same structure.
 

Q7. Name the three sub-atomic particles of an atom.
 Ans. The sub-atomic particles of an atom are Protons → Positively charged Electrons → Negatively charged Neutrons → No charge
 

Q8. Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?
 Ans. Atomic mass of He = 4u.
Atomic mass = No. of protons + No. of neutrons
∴ 4 = 2 + no. of neutrons.
∴ No. of neutrons = 4 − 2 = 2 Helium atom has 2 neutrons.
 

Q9. Write the distribution of electrons in carbon and sodium atoms.
 Ans. Carbon Atomic number = 6
∴ No. of protons = 6 and Number of protons = Number of electrons
∴ Distribution of electrons = K L 2 4
Sodium Atomic number = 11
∴ No. of protons = 11 = No. of electrons
∴ Distribution of electrons = K L M 2 8  1
 

Q10. If K and L shells of an atom are full, then what would be the total number of electrons in the atom?
 Ans. K shell can hold 2 electrons and L shell can hold 8 electrons.
When both the shells are full, there will be (8 + 2) 10 electrons in the atom.
 

Q11. How will you find the valency of chlorine, sulphur and magnesium?
 Ans. Valency is the combining capacity of an atom of an element.
Chlorine, Atomic Number = 17
∴ Protons = 17, Electrons = 17
∴ Distribution of electrons = K L M
                                             2 8 7

Chlorine needs 1 electron to complete its outermost orbit/shell.
∴ Its valency is −1 (gains 1 electron).
Sulphur, Atomic number = 16
∴ Protons = 16, Electrons = 16
∴ Distribution of electrons = K L M
                                             2 8 6
Sulphur needs 2 electrons to complete its outermost shell.
∴ Its valency is −2 (gains 2 electrons) Magnesium, Atomic number = 12
∴ Protons = 12, electrons = 12
∴ Distribution of electrons = K L M
                                             2 8 2
Magnesium needs to donate 2 electrons from its outermost shell to become stable.
∴ Its valency is +2 (donates 2 electrons).
 

Q12. If number of electrons in an atom is 8 and number of protons is also 8, then
 (i) What is the atomic number of the atom? and
 (ii) What is the charge on the atom?
 Ans. No. of electrons = 8 No. of protons = 8
(i) Atomic number = no. of protons = 8
(ii) As Electrons = Protons
∴ Atom is electrically neutral (No charge)

Q13. With the help of given Table 4.1, find out the mass number of oxygen and sulphur atom.

Table 4.1 : Composition of Atoms of the First Eighteen Elements with  Electron Distribution in Various Shells

Ans. Oxygen, No. of protons = 8
∴ No. of neutrons = 8
∴ Atomic number = 8
∴ Atomic mass number = P + N = 8 + 8 = 16
Sulphur, No. of protons = 16
∴ Atomic number = 16
No. of neutrons = 16
∴ Atomic mass number = P + N = 16 + 16 = 32

Q14. For the symbol H, D and T tabulate three sub-atomic particles found in each of them.
 Ans. (Protium) H → (₁¹H) Atomic number = 1, Mass number = 1
∴ No. of protons = 1
No. of electrons = 1
No. of neutrons = Nil
D → (₁²H) Atomic number = 1,
Mass number = 2
(Deuterium) ∴ No. of protons = 1
No. of electrons = 1
No. of neutrons = 1
T→ (₁²H) Atomic number = 1,
Mass number = 3
(Tritium)  ∴ No. of protons = 1
No. of electrons = 1
No. of neutrons = 2
 

Q15. Write the electronic configuration of any one pair of isotopes and isobar.
 Ans. Isotopes: Atoms of same element having same atomic number but different mass number.
∴ Electronic configuration of isotopes remain the same.

Isobars: Atoms of different elements with different atomic number but same mass number.

NCERT Textual Questions Solved 

Q1. Compare the properties of electrons, protons and neutrons.

Q2. What are the limitations of J.J. Thomson’s model of the atom?
 Ans. According to J.J. Thomson’s model of an atom, the electrons are embedded all over in the positively charged spheres. But experiments done by other scientists showed that protons are present only in the centre of the atom and electrons are distributed around it.
 

Q3. What are the limitations of Rutherford’s model of the atom?
 Ans. According to Rutherford’s model of an atom the electrons are revolving in a circular orbit around the nucleus. Any charged particle that revolves would undergo acceleration and radiate energy. The revolving electron would lose its energy and finally fall into the nucleus, thus the atom would be highly unstable.
But we know that atoms are quite stable.
 

Q4. Describe Bohr’s model of the atom.
 Ans. Bohr’s model of the atom
(1) Atom has nucleus in the centre.
(2) Electrons revolve around the nucleus.
(3) Certain special orbits known as discrete orbits of electrons, are allowed inside the atom.
(4) While revolving in discrete orbits the electrons do not radiate energy.
(5) These orbits or shells are called energy levels.
(6) These orbits or shells are represented by the letters K, L, M, N or the numbers n = 1, 2, 3, 4

Q5. Compare all the proposed models of an atom given in this chapter.
 Ans.

Thomson	Rutherford	Bohr
Sphere of positive charge	Sphere of positive charge in centre called nucleus. All mass of an atom resides in the nucleus	Positive charge in center called nucleus.
electron are spread randomly all over in the sphere	electron revolve around the nucleus in well defined orbit	elecron revolve in discrete orbit and do not radiate energy.
Positive charge = Neetive charge Atom in electrically nuteral	Size of nucleus is very small as compared to size of atom	the orbits were termed as energy shells labelled as K,L,M,N or n = 1,2,3,4 (numbered)

Q6. Summarise the rules for writing of distribution of electrons in various shells for the first eighteen elements.
 Ans. The rules for writing of distribution of electrons in various shells for the first eighteen elements are:
(i) The maximum number of electrons present in a shell is given by the formula−2n² 
∵ n = orbit number i.e., 1, 2, 3
∴ Maximum number of electrons in different shells are:

(ii) The maximum number of electrons that can be accommodated in the outermost orbit is 8.

(iii) Electrons are not accommodated in a given shell unless the inner shells are filled. (Shells are filled step-wise).
 

Q7. Define valency by taking examples of silicon and oxygen.
 Ans. Valency is the combining capacity of an atom.
Atomic number of oxygen = 8 Atomic number of silicon = 14
                                                           K L M
Electronic configuration of oxygen = 2 6 −              
Electronic configuration of silicon = 2 8 4
In the atoms of oxygen the valence electrons are 6 (i.e., electrons in the outermost shell). To fill the orbit, 2 electrons are required. In the atom of silicon, the valence electrons are 4. To fill this orbit 4 electrons are required.
Hence, the combining capacity of oxygen is 2 and of silicon is 4.
i.e.,               Valency of oxygen = 2
                     Valency of silicon = 4

Q8. Explain with examples:
 (i) Atomic number
 (ii) Mass number,
 (iii) Isotopes and
 (iv) Isobars.
 Give any two uses of isotopes.
 Ans. (i) Atomic number: The atomic number of an element is equal to the number of protons in the nucleus of its atom. e.g., Oxygen has 6 protons hence atomic no. = 6.

(ii) Mass number: The mass number of an atom is equal to the number of protons and neutrons in its nucleus.

Mass Number = number of protons + number of neutrons

Example: Carbon has 6 protons and 6 neutrons.

(iii) Isotopes: Isotopes are atoms of the same element which have different mass number but same atomic number.

(iv) Isobars: Isobars are atoms having the same mass number but different atomic numbers.

Both calcium and argon have same mass number but different atomic number.
Two uses of isotopes are: 
(a) An isotope of iodine is used in the treatment of goitre.
(b) An isotope of uranium is used as a fuel in nuclear reactors.
 

Q9. Na⁺ has completely filled K and L shells. Explain.
 Ans. Sodium atom (Na), has atomic number = 11
Number of protons = 11
Number of electrons = 11
∴ Electronic configuration of Na = K L M  
                                                      2 8   1
Sodium atom (Na) loses 1 electron to become stable and form Na+ ion. Hence it has completely filled K and L shells.
 

Q10. If bromine atom is available in the form of, say, two isotopes 7395Br (49.7%) and 3851Br (50.3%), calculate the average atomic mass of bromine atom.
 Ans. The average atomic mass of bromine atom

= 39.263 + 40.743
= 80.006 u

Q11. The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes  in the sample?
 Ans. Let the percentage of  be x and the percentage of  be 100 − x.

Q12. If Z = 3, what would be the valency of the element? Also, name the element.
 Ans. Z = 3, (i.e., atomic number → z)
∴ Electronic configuration = 2, 1
Valency = 1
Name of the element is lithium.
 

Q13. Composition of the nuclei of two atomic species X and Y are given as under   X Y
 Protons = 6 6 Neutrons = 6 8 Give the mass number of X and Y. What is the relation between the two species?
 Ans. Mass number of X
= Protons + Neutrons
= 6 + 6
= 12
Mass number of Y
= Protons + Neutrons
= 6 + 8 = 14
As the atomic number is same i.e., = 6.
[atomic number = number of protons].
Both X and Y are isotopes of same element.
 

Q14. For the following statements, write T for True and F for False.
 (a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons.
 (b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral.
 (c) The mass of an electron is about 1/2000 times that of proton.
 (d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.
 Ans. (a) False
(b) False
(c) True
(d) False
 

Put tick (P) against correct choice and cross (×) against wrong choice in questions 15, 16 and17.
 

Q15. Rutherford’s alpha-particle scattering experiment was responsible for the discovery of
 (a) Atomic nucleus (b) Electron (c) Proton (d) Neutron Ans. (a) Atomic nucleus

Q16. Isotopes of an element have
 (a) the same physical properties (b) different chemical properties (c) different number of neutrons (d) different atomic numbers.
 Ans. (c) different number of neutrons

Q17. Number of valence electrons in Cl⁻ ion are : (a) 16 (b) 8 (c) 17 (d) 18
 Ans. (b) 8
 

Q18. Which one of the following is a correct electronic configuration of sodium?
 (a) 2, 8 (b) 8, 2, 1 (c) 2, 1, 8 (d) 2, 8, 1
 Ans. (d) 2, 8, 1 

Q19. Complete the following table.

Ans.