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

c) should be the answer.