The no. of protons in the nucleus of the atom is equal to the atomic number (Z)

The no. of the neutrons is equal to the difference between the mass no. of the atom (A) and the Atomic no. (Z)

The mass no. of the atom (A) is equal to the sum of the no. of protons and neutrons in the nucleus.

$A \rightarrow$ Mass number $=$ Protons $+$ neutrons

$Z=$ Atomic no. $=$ No. of protons

$\therefore Z =$ Protons,

Neutrons $= A – Z$