An element's **mass number** is the **number of protons plus the number of neutrons**.

If we have the mass number and the atomic number of an element, we can find the number of protons, neutrons and electrons that element has:

*Argon has an atomic number of ***18** and a mass number of **40**.

The atomic number is 18 so it has **18 protons**.

Atoms have the same number of electrons as protons so it also has **18 electrons**.

The mass number (protons + neutrons) is 40. We know there are 18 protons, so the number of neutrons is 40 - 18, which is 22. There are **22 neutrons**.

## Comments

No comments have yet been made