1934 – UK
‘Hydrogen has three isotopes: hydrogen-1 (ordinary hydrogen: one proton), hydrogen-2 (deuterium: one proton, one neutron) and hydrogen-3 (tritium: one proton, two neutrons)’
They each have one single proton (z = 1), but differ in the number of their neutrons. Hydrogen has no neutron, deuterium has one, and tritium has two neutrons. The isotopes of hydrogen have, respectively, mass numbers of one, two, and three. Their nuclear symbols are therefore 1H, 2H, and 3H. The atoms of these isotopes have one electron to balance the charge of the one proton. Since chemistry depends on the interactions of protons with electrons, the chemical properties of the isotopes are nearly the same.
The lightest rare gas, helium, exists in nature in two forms – two isotopes
The usual form is represented as 4He, where the figure 4 stands for the number of nucleons in the atomic nucleus (two protons and two neutrons). In the unusual form, 3He, the atomic nucleus has only one neutron, so it is lighter. In helium that occurs naturally the heavier isotope is more frequent than the lighter one by a factor of about 10 million. That is why it is only in the last 50 years that it has been possible to produce large amounts of 3He, at nuclear power stations, for example. At normal temperatures the gases of the two isotopes differ only in their atomic weights.