Helium atom has an atomic mass of 4 units and 2 protons in its nucleus. How many neutrons are present in its nucleus?

Given data:

Atomic mass of Helium $\left(\mathrm{He}\right)$$=$Mass number of $\left(\mathrm{He}\right)=4$ units
Number of protons in Helium $\left(\mathrm{He}\right)$ nucleus$=2$

Formula used:

Mass number$=$Number of protons$+$Number of neutrons

Calculate the number of neutrons in Helium $\left(\mathrm{He}\right)$ nucleus:

Mass number$=$Number of protons$+$Number of neutrons

Thus,

Number of neutrons$=$Mass number$-$Number of protons

Number of neutrons$=4-2$

Number of neutrons$=2$

Thus, the number of neutrons in Helium $\left(\mathrm{He}\right)$ nucleus are $2$.