# What number is the sum of the protons and neutrons?

May 9, 2018

#### Explanation:

The sum of the number of protons and neutrons in an atom is called the mass number. For example:

Carbon-12 has 12 protons and 12 neutrons.

$12 + 12 = 24$, so the mass number of C-12 is $24$ (no units).