Question

calculate the atomic number of the element that has 12 neutrons and a mass number of 23.

Answer 1

atomic number
=number of protons
= mass number - number of neutron
=23-12
=11

Answer 2

Atomic number Z = number of electrons = number of protons  in an element. 
   Z = mass number M - number of neutrons
    Z = 23 - 12 = 11