When atoms of the same element have different mass numbers, what are they known as?

iso means the same like in triangles (isosceles) so the atoms are the same element but different in mass. Since the isotopes have the same number of protons and electrons the isotopes have much the same chemical behavior. Since the isotopes have different numbers of neutrons the nuclear behavior differs.

One of the best known isotopes is ## C_6^14## Carbon fourteen. Carbon fourteen is absorbed by plants and used in the plant exactly as the most common isotope ##C_6^12##. The difference is that when the plant dies and stops absorbing Carbon fourteen the percentage of Carbon 14 in the plant starts to decrease as the nuclear unstable Carbon 14 breaks down. Carbon 12 is a nuclear stable atom. Carbon 14 is an nuclear unstable atom.