Atomic mass - The sum of the weights of both the neutrons and the protons in the atom.

Electron - A small negatively charged particle that surrounds the nucleus with a mass about 1/1800 that of the proton . Beta particles are energetic electrons ejected from a radioactive nucleus.

Element - the most basic physical substance composed of all the same type of atoms. Each atom will have the same number of protons. The number of neutrons can differ.

Isotope - Atoms with the same number of protons, but differing in the number of neutrons present in the nucleus. Most elements have more than one isotope.

Neutron - An electrically neutral particle found in the nucleus with a mass almost that of the proton. In the fission process, neutrons are liberated.

Nucleus - The densely packed kernel of the atom containing protons and neutrons. The diameter of the nucleus is 100,000 to 200,000 smaller than the whole atom.

Photon - The smallest unit of light. The photon is often described as a electromagnetic wave or wave packet. Light photons from red to blue in the visible spectrum have increasing energy. X-rays and gamma rays are energetic photons with thousands to millions of times the energy of light photons.