Question

An atom has a mass number of 24 and 13 neutrons. What is the atomic number of this atom?

Answer 1

A=Mass number=24
N=neutrons=13
Z=atomic number.

A=Z+N

24=Z+13
Z=24-13
Z=11

The atomic number is 11, and this atom is sodium. 

Answer 2

11,the numbers of protons and electrons are always equal in an atom,the numbers of protons tell us about atomic number of element
mass number=no. of P+no. of N
here,24
no. of P(atomic number)=24-13=11