If the atomic mass of an atom is 34 and the atom contains 13 protons, how many neutrons does the atom contain?

=21 neutrons

Explanation:

The atomic mass of an atom is the total sum of the contents of the nucleus (protons and neutrons).

Thus atomic mass = no. of neutrons + no. of protons

Number of protons = The atomic mass-number of protons

= 34 - 13 = 21

That would be 21 neutrons (34-13=21).

Atom mass is the number of protons+number of neutrons.