Exercise 8.12
An atom has protons and a mass number of 197. Calculate (i) the number of neutrons, and (ii) the number of electrons.
NCERT’s answer
(i)
Neutrons $\displaystyle 118$ (ii) Electrons $\displaystyle 79$
(i) $\displaystyle 118$ neutrons (ii) $\displaystyle 79$ electrons
Protons \(\displaystyle = 79\), so \(\displaystyle Z = 79\); mass number \(\displaystyle A = 197\).
(i) \(\displaystyle \text{neutrons} = A - Z = 197 - 79 = 118\)
(ii) The atom is electrically neutral, so electrons \(\displaystyle =\) protons \(\displaystyle = 79\).
(\(\displaystyle Z = 79\) is gold, symbol Au from the Latin aurum.)