# An atom has a mass number of 25 and 13 neutrons. What is the atomic number of this atom?

May 13, 2018

The atomic number is $12$, which is the atomic number for magnesium.

#### Explanation:

The mass number of an isotope is the sum of the protons and neutrons in its atomic nuclei.

If we know the mass number $\left(\text{A}\right)$ and the number of neutrons $\left(\text{N}\right)$, we can calculate the atomic number $\left(\text{Z}\right)$ by subtracting the number of neutrons from the mass number.

$\text{A"-"N"="Z}$

$25 - 13 = 12$

The atomic number of the element is $12$. Atomic number is the number of protons in the nuclei of all atoms of an element. The element with the atomic number 12 is magnesium.

This is $\text{^25"Mg}$, or magnesium-25, which can be abbreviated as Mg-25.