The word "ion" is from Greek ion, present participle of ienai "go", thus "a goer". "Anion" and "cation" mean "up-goer" and "down-goer", and "anode" and "cathode" are "way up" and "way down" (hodos=road, way).
For single atoms in a vacuum, there are physical constants associated with the process of ionization. The energy needed to remove electrons from an atom is called the ionization energy, and the equivalent electrical potential -- i.e. the energy divided by a single electron charge -- is known as the ionization potential. These terms are also used to describe ionization of molecules and solids, but the values are not constant because ionization can be affected by the local chemistry, geometry, and temperature.
Ionization energies decrease down a group of the Periodic Table, and increase left-to-right across a period. These trends are exact opposite of the atomic radius periodic trends. Electrons in smaller atoms are attracted more strongly to the nucleus, therefore the ionization energy is greater. In larger atoms, the electrons are not held as strongly so the required ionization energy is lesser.
|Successive Ionization Energies in kJ/mol|
The first ionization energy is the energy required to remove one electron, the second to remove two electrons, and so on. The successive ionization energies are always greater than the previous, and a certain nth ionization energy will be significantly larger than the rest. For this reason, ions tend to form with in certain ways. For example, sodium is found as Na+, but not usually Na2+ due to the large amount of ionization energy required. Likewise, magnesium is found as Mg2+, but not Mg3+ and aluminum may exist as an Al3+ cation.