9.1 Atomic structure notation

Much of chemistry focuses on chemical reactions which is governed by the behavior of electrons. Nuclear chemistry focuses on nuclear reactions which involve a change in an atom’s nucleus (referred to as a nuclide). Recall the notation used to describe isotopes which gives the

atomic number (number of protons)
mass number (number of protons + neutrons)

The same notation is used to describe a nuclide. For example, the notation for the most common isotope of helium is

\[^{4}_{2}\mathrm{He}\]

where “2” is the number of protons (directly from the periodic table) and the mass number is “4” indicating this isotope of helium has 2 neutrons. The notation for carbon-14 would be

\[^{14}_{\phantom{1}6}\mathrm{C}\] indicating that carbon-14 has 8 neutrons.

Protons and neutrons are collectively called nucleons. These subatomic particles are held together by a the very powerful strong nuclear force that is strong over very short distances (about 1 fm) and rapidly decays in strength beyond around 2.5 fm. There is an incredible amount of energy here that holds the nucleus together and is called the nuclear binding energy.