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An ion is an atom or group of atoms with a net electric charge. A negatively charged ion, which has gained one or more electrons, is known as an anion, for it is attracted to anodes, and a positively charged ion, which has lost one or more electrons, is known as a cation (pronounced cat eye on), for it is attracted to cathodes.
In physics or chemistry, an ion is an electrically charged molecule or atom that has gained or lost electrons from its normal complement in a process known as ionization.
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 he won the Nobel Prize in Chemistry in 1903 for the same dissertation.
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, or 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 radiusAtomic radius is the distance from the atomic nucleus to the outmost stable electron orbital in a atom that is at equilibrium and is measured in picometers or Angstroms. Atomic radii are called covalent radii (a reference to the types of bonds formed) whe 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 |
|---|---|---|---|---|---|---|---|
| NaSodium is the chemical element in the periodic table that has the symbol Na Natrium in Latin) and atom number 11. Sodium is a soft, waxy, silvery reactive metal belonging to the alkali metals that is abundant in natural compounds (especially halite). | 496 | 4,560 | |||||
| MgMagnesium is the chemical element in the periodic table that has the symbol Mg and atomic number 12. Magnesium is the eighth most abundant element and constitutes about 2% of the Earth's crust, and it is the third most plentiful element dissolved in seawa | 738 | 1,450 | 7,730 | ||||
| AlAluminium (or aluminum in North American English) is the chemical element in the periodic table with the symbol Al and atomic number 13. A silvery and ductile member of the poor metal group of elements, aluminium is found primarily as the ore bauxite and | 577 | 1,816 | 2,744 | 11,600 | |||
| SiSilicon is the chemical element in the periodic table that has the symbol Si and atomic number 14. A tetravalent metalloid, silicon is less reactive than its chemical analog carbon. It is the second most abundant element in the Earth's crust, making up 25 | 786 | 1,577 | 3,228 | 4,354 | 16,100 | ||
| PPhosphorus is the chemical element in the periodic table that has the symbol P and atomic number 15. A multivalent, nonmetal of the nitrogen group, phosphorus is commonly found in inorganic phosphate rocks and in all living cells but is never naturally fo | 1,060 | 1,890 | 2,905 | 4,950 | 6,270 | 21,200 | |
| SSulfur (or sulphur see spelling) is the chemical element in the periodic table that has the symbol S and atomic number 16. An abundant tasteless odorless multivalent non-metal, sulfur is best known as yellow crystals and occurs in many sulfide and sulfate | 999 | 2,260 | 3,375 | 4,565 | 6,950 | 8,490 | 11,000 |
| Cl | 1,256 | 2,295 | 3,850 | 5,160 | 6,560 | 9,360 | 11,000 |
| Ar | 1,520 | 2,665 | 3,945 | 5,770 | 7,230 | 8,780 | 12,000 |
| Successive Ionization Energies in k J/ mol | |||||||
The first ionization energy is the energy required to remove an electron from an isolated uncharged atom, the second to remove an electron from an isolated +1 ion, 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 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 aluminium may exist as an Al3+ cation.