Question

Answer the following questions.

(a) What is the difference between the models of the atom proposed by Thomson and by Rutherford?

(b) What is the characteristic property of the element hydrogen?

(c) What is meant by valency?

(d) What is meant by atomic mass number?

(e) Name the fundamental particles in an atom.

Answer 1

(a) The difference between Thomson's atomic model and Rutherford's atomic model is as under:

Basis of difference		Thomson's model		Rutherford's model
Definition		This theory proposes that the atom has a positively−charged part like the red part of water melon, and in this part negatively−charged particles are embedded like the seeds.		This model states that the nucleus at the centre of the atom has positive charge while the negatively−charged electrons revolve around the nucleus.

(b) Hydrogen atom does not contain neutrons in its nucleus. It has only 1 proton and 1 electron, so its atomic mass number and atomic number both are 1.

(c) Valency is defined as the number of electrons an atom requires to lose, gain or share to complete its valence shell. It can be calculated using the number of valence electrons.

If,

Number of valence electrons ≤ 4: valency = number of valence electrons

Number of valence electrons > 4: valency = 8 − number of valence electrons

For example, for element ‘aluminium’ having electronic configuration 2, 8, 3, the valency is 3. However, for element ‘oxygen’ having electronic configuration 2, 6, the valency is 8 − 6 = 2.

(d) Atomic mass number is equal to the sum of the number of protons (p) and number of neutrons (n) in the nucleus of an atom. It is represented by A.

Atomic mass number = Number of protons + Number of neutrons

A = p + n

Example: A sulphur atom contains 16 protons and 16 neutrons. Its atomic mass number, A = 16 + 16 = 32.

(e) There are three types of fundamental particles present in an atom:

1. Electron

2. Proton

3. Neutron