Question

The number of electrons in an element $X$ is 15 and the number of neutrons is 16. The element can be represented as :

• A. $\begin{array}{c}31\\ 15\end{array}X$
• B. $\begin{array}{c}31\\ 16\end{array}X$
• C. $\begin{array}{c}16\\ 15\end{array}X$
• D. $\begin{array}{c}15\\ 16\end{array}X$

Answer 1

The correct option is A $\begin{array}{c}31\\ 15\end{array}X$

${}_Z^{A}X$, where A stands for the atomic mass number which is the sum of protons and neutrons in the atom of an element and Z stands for an atomic number of the element which is the number of electrons in an atom of an element.

Atomic mass $=$ Number of electrons + Number of neutrons

Atomic number $=$ Number of protons $=$ Number of electrons

Therefore, atomic mass $=15+16=31$ and atomic number is $=15$.