Answer:
They have different amounts of neutrons.
They have different mass numbers.
Explanation:
Carbon-12 and carbon-13 are two isotopes of the element carbon. The difference between carbon-12 and carbon-13 is the number of neutrons in each atom. Atoms of both isotopes of carbon contain 6 protons. Atoms of carbon-12 have 6 neutrons, while atoms of carbon-13 contain 7 neutrons. And because the mass number is the sum of protons and neutrons in the nucleus then will be different too. Atoms of carbon-12 have the mass number equal to 12 a.m.u. while atoms of carbon-13 have the mass number equal to 13 a.m.u. .
a.m.u. - atomic mass units
