Structure of Atom – Text Book Solutions

Q. 1. What are canal rays?

Ans. The positively charged radiations discovered by E. Goldstein in a gas discharge are called canal rays.

Q. 2. If an atom contains one electron and one proton, will it carry any charge or not?

Ans. No, the atom will not carry any charge. It will be neutral.

Q. 3. On the basis of Rutherford’s model of an atom, which sub-atomic particle is present in the nucleus of an atom?

Ans. According to Rutherford’s model, proton is present in the nucleus of an atom.

Q. 5. On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole.

Ans According to J.J. Thomson’s model, an atom is a sphere in which positive charge is uniformly distributed. The electrons are embedded into it in such a way to give most stable electrostatic arrangement. This model explains the neutrality of the atom because equal number of electrons and positively charged particles are embedded together.

Q. 6. What do you think would be the observation if the a—particle scattering experiment is carried out using a foil of a metal other than gold?

Ans. Rutherford used a very thin foil of gold (0.00006 in thick) because it is the most malleable metal. The a-particles were much smaller than the gold atoms in the foil. But if any other metal would have been used, there I would not have been much size difference between u-particles and metal atoms and therefore there would not have been any penetration.

Q. 7. Name the three sub-atomic particles of an atom.

Ans. The three sub atomic particles of an atom are electron, proton and neutron.

Q. 8. Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?

Ans. Number of neutrons = Atomic mass — Number of protons

Therefore, the number of neutrons in the atom = 4—2 = 2

Q. 9. Write the distribution of electrons in carbon and sodium atoms.

Ans. (a) Electronic distribution in carbon atom
Atomic number of carbon 6
Number of electrons = 6
Distribution of electrons: K = 2, L= 4

(b) Distribution of electrons in sodium atom
Atomic number of sodium atom = 11
Number of electrons in sodium atom = 11
Distribution of electrons in sodium atom: K = 2, L = 8, M = 1

Q. 10. If K and L shell of an atom are full, then what would be the total number of electrons in the atom?

Ans.. The maximum capacity of K shell is 2 electrons and L shell can accommodate maximum 8 electrons in it. Therefore, there will be ten electrons in the atom.

Q. 11. How will you find the valency of chlorine, sulphur and magnesium?

Ans. (i) To determine the valency of chlorine atom
Atomic number of chlorine = 17
Number of electrons in chlorine atom = 17
Distribution of electrons in chlorine atom: K 2, L = 8, M = 7
Valency of chlorine atom = 8— 7 = Valency of chlorine atom is 1.
(ii) To determine the valency of sulphur atom
Atomic number of sulphur atom = 16
Number of electrons in sulphur atom = 16
Distribution of electrons in sulphur atom: K=2, L= 8, M = 6
Valency of sulphur atom = 8—6 = 2

Valency of sulphur atom is 2.

(iii) To determine the valency of magnesium atom
Atomic number of magnesium = 12
Number of electrons present in magnesium atom = 12
Distribution of electrons in magnesium atom: K= 2, L = 8, M = 2
Valency of magnesium atom is 2 since there are only two electrons in the outermost shell.

Q. 12. 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. (1) Atomic number will be 8.
(ii) Charge would be zero because number of the positive charges (protons) and negative charges (electrons) is equal.