Assertion (A): The mass of the total number of protons and neutrons is a measure of the approximate mass of an atom.
Reason (R): The mass of an electron is approximately negligible.

(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true and R is false.
(d) A is false and R is true.

[1 Mark]

Solution:

The correct answer is option (a): Both A and R are true and R is the correct explanation of A.

[1 Mark]

Proton, neutron and electron are the three sub-atomic particles.
The mass of a proton is approximately 1 u.
The mass of a neutron is approximately 1 u.
The mass of an electron is approximately negligible.

Since the mass of electron is negligible, therefore the total mass of an atom is the sum of the masses of protons and neutrons. Therefore, Both A and R are true and R is the correct explanation of A.