What is the mass number of an atom that has 5 protons and 7 neutrons? I give brainliest.

The atomic mass of an atom that has 5 protons and 7 neutrons would be 12.

Explanation:

In all cases, the number of protons added to the number of neutrons equate to the mass of an atom. Electrons do have mass, but it's so miniscule and insignificant that it doesn't contribute to the overall mass of the atom, so there's no need to count them.  

Bonus Information:

DO NOT CONFUSE MASS WITH CHARGE! Protons and neutrons are the only particles that contribute to the mass of an atom. Meanwhile, protons (+) and electrons (-) are the only particles that have a charge. As it says in its name, neutrons are neutral or have no charge.

Remember:

C = p - e

M = p + n

C = atomic charge, M = atomic mass, p =protons, n = neutrons, e = electrons

I hope this helps!