# How many neutrons are in an atom of "_86^222Rn?

Mar 5, 2017

$136$

#### Explanation:

The ""_86^222Rn atom has a mass number (at the top) of $222$ and an atomic number (at the bottom) of $86$.

The mass number is the number of protons and the number of neutrons, each with mass of $1$, added together. The atomic number is just the number of protons.

Therefore, we can find out the number of neutrons by simply subtracting the two:

$222 - 86 = 136$ neutrons.