Question:
The number of electrons in an element $X$ is 15 and the number of neutrons is 16 . Which of the following is the correct representation of the element?
(a) ${ }_{15}^{31} \mathrm{X}$
(b) ${ }_{16}^{31} \mathrm{X}$
(c) ${ }_{15}^{16} \mathrm{X}$
(d) ${ }_{16}^{15} \mathrm{X}$.
Solution:
(a) Mass no. $=$ no. of electrons (protons) $+$ no. of neutrons $=15+16=31$.
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