isotope - 1913, coined by British chemist Frederick Soddy, from Greek  isos "equal" + topos "place;" so called because despite the different atomic weights, the various forms of an element occupy the same place on the periodic table.

    Matter's fundamental unit is the atom which is comprised of electrons, protons, and neutrons. An atom must have as many electrons as it has protons, but the number of neutrons is only approximately the same as the number of protons.

   Two atoms that have the same number of protons and electrons but differ in their number of neutrons are called isotopes.

    ¹H == hydrogen atom with only a proton in its nucleus

    ²H == deuterium, hydrogen atom with a proton and neutron in its nucleus

    ³H == tritium, hydrogen atom with a proton and 2 neutrons in its nucleus

Isotopes behave identically in chemistry because they are two slightly different versions of the same element (hence occupy the same place on the periodic table). However, they can have very different properties in nuclear reactions.