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. ${}_{16}{}^{15}\mathrm{X}$

Answer 1

The correct option is A

${}_{15}{}^{31}\mathrm{X}$

Representation of the element:

In the element, both the mass number and atomic number can be indicated on the symbol of an element as follows: ${}_{\mathrm{Z}}{}^{\mathrm{A}}\mathrm{X}$

Where,

X is the symbol of the element.

Z is the atomic number.

A is the mass number.

Explanation:

Given:

Number of electrons = 15

Number of neutrons = 16

The atomic number of the element is equal to the number of electrons and protons.

Therefore, Z = 15.

$\begin{array}{rcl}\mathrm{The}\mathrm{atomic}\mathrm{mass}& =& \mathrm{number}\mathrm{of}\mathrm{protons}+\mathrm{number}\mathrm{of}\mathrm{neutrons}\\ & =& 15+16\\ & =& 31\end{array}$

Hence, option a) ${}_{15}{}^{31}\mathrm{X}$ is the correct answer.