Radiation Safety Refresher

Section 4: Basic radiation concepts (cont.)

Nuclear structure

First we need to discuss the Bohr model of the atom. If you know all this already, bear with us. If you want a more in-depth discussion, follow the link by clicking on the picture of the atom to the right.

You may remember that the atom is made up of a nucleus of positively-charged protons and neutral neutrons, orbited by a shell of negatively-charged electrons. Physicists have special terms when referring to the inhabitants of the nucleus. The number of protons is known as the "atomic number", or "Z number" (don’t ask me why it’s called "Z"). The number of protons determines the element. The number of neutrons is called the "neutron number", or "N number" (now, that makes more sense).

Lastly, the number of nucleons (protons and neutrons) is called the "atomic mass number", or "A number".

Therefore, A = Z + N

nuclide

We symbolize a nuclide in this manner.

To the left of the element symbol (X) the atomic number (A) appears on top and the number of protons (Z) on the bottom.

C14

For example, "C-14", an unstable form of carbon, looks like this:

We know C-14 because of its use in Anthropology or Geology for "carbon dating" organic matter. Because carbon always has six protons, you are likely to see its isotopes symbolized without the "Z" number.

Carbon-12 is another isotope of carbon. "Isotopes" are different nuclides with the same number of protons but different numbers of neutrons. In this case, 12C has six neutrons and 14C has eight. Carbon-12 is a stable isotope, but carbon-14 is unstable (a radioisotope, or radionuclide).