Isotopes of Hydrogen

Hydrogen offers a simple example of isotopes. Figure 4 shows three forms of the element: The first is just regular hydrogen, designated at H or 1H, which has one proton and no neutrons. [It has a name, protium, but it's only used by real physics nerds or trivia freaks.]

The second form, deuterium or 2H, is known as "heavy hydrogen." It has the obligatory single hydrogen proton, but also has a neutron in its nucleus. [Deuterium is the form of hydrogen that makes "heavy water," the substance on which many World War II spy novels are based. These stories are based on a factual February 1943 raid on the heavy water concentration plant operated by the Nazis in Vemork, Norway. While an earlier attempt resulted in disaster - all thirty-four commandos were either killed in glider crashes or executed by the Germans - the second raid was a textbook example of guerrilla warfare and bravery. Disabling the Vemork plant was a major setback for the Axis A-bomb program.]

Finally, the form of hydrogen known as tritium, or 3H, is seen to have two neutrons.

All three forms are known as isotopes with 1H making up 99.98% of the total hydrogen that we know about - and presumably in the universe. Both "regular" hydrogen and deuterium are stable isotopes, meaning that they don't change over time.

But tritium is anther story: it is an unstable isotope that will eventually decay or disintegrate into stable 1H. [All tritium undergoes "beta-decay" in which the neutron changes into a proton and an electron - a feat of amazing atomic legerdemain.] It is this process - where atoms go to pieces - that we normally call radioactivity.