Isotopes

• Isotopes are atoms of the same element with the same proton number but a different nucleon number. In other words, the same number of protons but a different number of neutrons.
• Chlorine, for example, has two common forms. Some atoms have 18 neutrons and some have 20. Both have 17 protons, so both are chlorine, but their masses differ. We write them ${}^{35}_{17}\text{Cl}$ and ${}^{37}_{17}\text{Cl}$.

• Chemistry depends on electrons. The number of electrons is set by the number of protons, and isotopes have the same number of protons. So they have the same electrons and behave the same way in chemistry.
• The only difference is in the nucleus. The extra neutrons change the mass, and can change whether the nucleus is stable or radioactive. That is a physical difference, not a chemical one.
• This is why carbon-14 dating works. Carbon-14 reacts exactly like ordinary carbon-12, so living things absorb it the same way. But carbon-14 is radioactive and slowly decays, while carbon-12 does not. (9702/11/M/J/24 Q37)

Isotopes: same protons (same element, same chemistry), different neutrons (different mass, and maybe different stability). The neutrons are the only thing that changes.