# How is the mass number written?

##### 1 Answer
Jun 13, 2018

On the top left side in an isotope...

#### Explanation:

An isotope is usually fully written in the form of:

""_Z^AX

where:

The mass number is also written at the top of an element's position on the periodic table. It has no units, as the normal units are presumed to be atomic mass units or $\text{u}$.

Note that $1 \setminus \text{u"=1.66*10^-27 \ "kg}$.