Atoms are unstable unless they have a similar number of protons and neutrons. to make the carbon atom stable, add 6 neutrons to it. what's the mass number now?

Mass number of carbon is 12.

Explanation:

Carbon atom is defined as 6 atoms in a nucleus, and the mass number is the total number of protons and neutrons in the nucleus of an atom.  

Before any neutrons are added to the carbon atom, its mass number is 6; after 6 neutrons are added, the total number of protons and neutrons is now 12—the mass number becomes 12.

come closer and become far apart and become steam/gas

at right angles to each other