Elements: Each element has a fixed number of positively charged protons in its nucleus and an equal number of electrons orbiting the nucleus. For example, hydrogen (H) has one proton and one electron, but lead (Pb) has 82 protons and 82 electrons. There are about 115 known elements of which 82 are naturally abundant.

Isotopes: The nucleus contains both protons and neutrons. An element has a fixed number of protons but may exist with various numbers of neutrons. The sum of the protons and neutrons is the mass number. For example, helium exists as 3He(2 protons and one neutron) or as 4He (2 protons and 2 neutrons). The two forms of helium are called isotopes of helium. Isotopes of an element have the same chemical properties but different weights. Some elements have many isomers. Tin (Sn) has about 38 known isotopes.

Half-life: The half-life of an isotope is the amount of time it takes for half of the atoms to decay into a more stable form. Naturally abundant isotopes exist around us because their half-lives are longer than the age of the earth. Uranium 238 (238U) has a half-life of 4.5 billion years so it is naturally abundant. Most isotopes have short half-lives and must be produced in the laboratory to study or use. For example, cobalt 60 (60Co) has a half-life of 5.3 years and is made in a reactor. 60Co is used for radiation therapy of cancer patients. Over 3500 isotopes are known, and most are merely laboratory curiosities.

Isomers: In some cases more than one form of an isotope can exist. These different forms of the isotope have the same number of protons and neutrons but different half-lives. They are different because the protons and neutrons can arrange themselves in different ways. There is an isomer of 60Co with a half-life of 10 minutes.

Decay Modes: Isotopes try to decay to more stable isotopes. They may do this by beta decay emitting an electron and converting a neutron to a proton (B- decay), emitting a positron (anti-electron) and converting a proton to a neutron (B+ decay), or capturing an atomic electron and converting a proton to a neutron (electron capture decay). Beta decay also emits a nearly invisible neutrino. Some isomers decay by emitting a gamma-ray (high energy photon or light). Other isotopes decay by alpha decay, emitting an alpha particle which is really a 4He nucleus. Heavy elements occasionally decay by spontaneous fission, dividing in half. An isotope can decay in several possible ways often emitting gamma-rays as loses energy.