Ion
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An ion is an elementary particle or system of elementary particles with a net electric charge.
A neutral atom contains an equal number of protons in the nucleus and electrons in the electron shell. A negatively charged atomic ion, which has gained one or more electrons, is known as an anion, and a positively charged atomic ion, which has lost one or more electrons, is known as a cation. The process of removing or adding charges to a neutral particle is called ionization, the inverse processes are called recombination and detachment. Note that neutral elementary particles cannot be ionized, as they contain no constituent charged particles that can be removed. The formation of free negatively charged atomic ions is non-trivial because an additional electron doesn't experience a Coulomb attraction towards the neutral atom. It is nevertheless possible in many cases, see negative atomic ion.
Ionized atoms or molecules are denoted by superscripting with the number of electrons lost or gained (if more than one) and the sign (+ or −) of the electric charge, e.g. H+ or O2−.
Metals tend to form cations while non-metals tend to form anions e.g. sodium forms cations of Na+ while chlorine forms anions of Cl-.
Ionization energy
The energy required to produce a cation by stripping an electron from a more nearly neutral atom is the ionization energy. More generally, the n-th ionization energy of an atom is the energy required to strip it of its n-th electron after the first n − 1 have already been removed.
Each successive ionization energy is markedly greater than the last. Particularly dramatic increases occur after any given block of atomic orbitals is exhausted. For this reason, ions tend to form in ways that leave them with full orbital blocks. 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 aluminium may exist as an Al3+ cation.
History
Ions were first theorized by Michael Faraday around 1830, to describe the portions of molecules that travel either to an anode or to a cathode. However, the mechanism by which this was achieved was not described until 1884 by Svante August Arrhenius in his doctoral dissertation to the University of Uppsala. His theory was initially not accepted (he got his degree with a minimum passing grade) but his dissertation won the Nobel Prize in Chemistry in 1903.