Answer :

An atom is called the mass number. 
When an atom sticks together it's called: Residual electromagnetic force.

The reason they stick together is because: When the atoms are electrically neutral, but the electrons in one are attracted to the protons in another, and vice versa. That's what causes the Residual electromagnetic force.

I know this because I did some research online.

Hope I helped!

- Amber  

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