All atoms of one element have the same number of protons, but they do not all have the same number of neutrons. Atoms of the same element with different numbers of neutrons are called isotopes. Changing the number of neutrons changes the mass of the nucleus, so isotopes of an element have different masses.
The mass number of an atom is the total number of protons and neutrons in its nucleus. Carbon-12 has 6 protons and 6 neutrons, so its mass number is 12. Carbon-14 also has 6 protons, so it is still carbon, but its mass number is 14, so it has 8 neutrons. Carbon-12 and carbon-14 are two isotopes of carbon, and carbon-14 has the greater mass.
Isotopes of an element have the same number of electrons, so they react in the same way chemically. Some isotopes have nuclei that are unstable, and these are radioactive.
