Symbol of nucleus with atomic number 18 and mass number 40 is written as

Atomic number: The number of protons in the nucleus of an atom. It is denoted by letter Z. 

Mass number: The number of prolong + the number of neutrons. It is denoted by letter A.

Atomic mass number $\left(A\right)=Z+n$

Number of protons atomic number  ${}_{18}A{r}^{40}$ $\left(A\right)=Z+n$