Life isn't as simple as hydrogen and helium make it appear. If you look at the Periodic Table, the masses of hydrogen and helium are not simple whole numbers, as you would expect if you added the number of protons with the number of neutrons. Where do the decimal numbers come from?
|Explore the three isotopes of hydrogen by scrolling your mouse over the following isotopes.|
Atoms don't always contain the same number of neutrons, regardless of the number of protons, nor are the number of protons and neutrons always equal. Hydrogen, for example, shown above, is found in three forms, all with only one proton. Protium, the version of hydrogen we are most familiar with, has one proton and one electron. Deuterium, another version of hydrogen, has one proton, one electron, and one neutron! Another version, tritium, has one proton, TWO neutrons, and one electron.
These different versions, with varying numbers of neutrons are called isotopes. Isotopes of a given element have the same number of protons, but different number of neutrons. It's the different number of neutrons that gives rise to different masses of the same element.