Calculating the ionization energy of an atom constitutes a part of modern physics that underlies many modern technologies. An atom consists of a central nucleus that contains positively charged protons and a number of neutrons specific to the given atom. A number of negatively charged electrons orbit the nucleus at various distances. The energy required to remove the lowest orbiting electron from the influence of the central protons is the ionization energy. Danish physicist Niels Bohr first calculated this energy for hydrogen in 1913, for which he won the Nobel Prize.

Determine what atom you want to use for calculating the ionization energy. Identify the value of "Z" for the atom using a periodic table. (Another name for the number Z is the atomic number.) The value for Z appears above the symbol for the atom. For example, Z equals 1 for hydrogen.

Decide how many electrons the atom contains. This number is the same as Z unless the atom has already lost some electrons.

Calculate the ionization energy, in units of electron volts, for a one-electron atom by squaring Z and then multiplying that result by 13.6.

For atoms with more than one electron, arrive at the ionization energy, in units of electron volts, by first subtracting one from Z, squaring the answer, and finally multiplying by 13.6.