Ion : Cation 

Negatively charged ions are known as anions (which are attracted to anodes) and positively charged ions are called cations (which are attracted to cathodes).

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.

Element	First	Second	Third	Fourth	Fifth	Sixth	Seventh
Na	496	4560
Mg	738	1450	7730
Al	577	1816	2744	11,600
Si	786	1577	3228	4354	16,100
P	1060	1890	2905	4950	6270	21,200
S	999	2260	3375	4565	6950	8490	11,000
Cl	1256	2295	3850	5160	6560	9360	11,000
Ar	1520	2665	3945	5770	7230	8780	12,000
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.

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