Write the atomic number of an element which has mass number $23$ and number of neutrons $12$ in the nucleus.

(11) The atomic number $(Z)$ is equal to the number of protons in the nucleus of an atom.
The mass number $(A)$ is the sum of the number of protons and the number of neutrons in the nucleus.
$A = \text{Number of Protons} + \text{Number of Neutrons}$
Given:
Mass number $(A)$ = $23$
Number of neutrons = $12$
Substituting the values:
$23 = \text{Number of Protons} + 12$
Number of Protons = $23 - 12 = 11$
Since the number of protons is equal to the atomic number $(Z)$, the atomic number is $11$.