Jump to content

Ion

From Tilcayo Cats, the lean encyclopaedia by Tilcayo Knowledge Systems
Tilcayo Cats original · British Bees · Sources and editorial assessment · Originals retention rule

An ion is an atom or group of atoms that carries an electric charge because it has gained or lost electrons. An ion with a positive charge is called a cation, and one with a negative charge is called an anion.[1][2] Ions have physical and chemical properties that are very different from those of the elements they are formed from.[3]

How ions form

A cation forms when an atom loses one or more electrons, and an anion forms when an atom gains one or more electrons.[2][4] An ion formed from a single atom is called a monatomic ion. Its charge is written as a small raised number and sign after the symbol: a magnesium atom that loses two electrons becomes Mg2+, and a bromine atom that gains one electron becomes Br−.[5]

The energy needed to remove an electron from an atom in a gas is called its ionisation energy.[6] Sodium easily loses one electron to form Na+, but removing a second electron would take almost ten times as much energy, so sodium does not normally form a 2+ ion.[2] In the same way, magnesium's third ionisation energy is much larger than its first two, so magnesium forms 2+ ions.[7]

Non-metals in groups 5, 6 and 7 of the periodic table tend to form anions, such as oxide (O2−), bromide (Br−) and phosphide (P3−). Hydrogen can form both H+ and H−. The noble gases are unreactive and do not form ions.[8]

Size of ions

Because forming a cation removes electrons, cations are always smaller than the atoms they come from.[9] Anions are larger than their parent atoms: a fluorine atom has a radius of 71 picometres, but a fluoride ion has a radius of 133 picometres.[9]

Ionic compounds

In ionic bonding, electrons are transferred from one atom to another, and the ions that result are held together by electrostatic attraction. Ionic compounds do not exist as separate pairs of ions under ordinary conditions; instead the ions are arranged in a three-dimensional framework.[9] In sodium chloride each ion is surrounded by six ions of the opposite charge.[9]

The properties of ions can be very different from those of the element. Magnesium metal is shiny and burns in air with a bright white flame, but magnesium ions are colourless and are found in most drinking water.[3]

History

The word ion was coined by Michael Faraday, with William Whewell, for the parts of a compound that are released at the electrodes during electrolysis. Faraday's ions were not quite the same as the ions in the later theory of Svante Arrhenius.[10] In 1884 Arrhenius wrote a thesis on electrical conductivity in solutions which suggested that some substances split up (dissociate) into ions when they dissolve. The thesis was given only a fourth-class pass.[11][12]

See also

References

  1. ↑ Young, Susan M.; Vining, William J.; Day, Roberta; Botch, Beatrice (2018). General Chemistry: Atoms First. Boston: Cengage Learning. p. 40. ISBN 978-1-337-61229-6. {{cite book}}: |access-date= requires |url= (help)
  2. ↑ 2.0 2.1 2.2 Young, Susan M.; Vining, William J.; Day, Roberta; Botch, Beatrice (2018). General Chemistry: Atoms First. Boston: Cengage Learning. p. 105. ISBN 978-1-337-61229-6. {{cite book}}: |access-date= requires |url= (help)
  3. ↑ 3.0 3.1 Young, Susan M.; Vining, William J.; Day, Roberta; Botch, Beatrice (2018). General Chemistry: Atoms First. Boston: Cengage Learning. p. 42. ISBN 978-1-337-61229-6. {{cite book}}: |access-date= requires |url= (help)
  4. ↑ Young, Susan M.; Vining, William J.; Day, Roberta; Botch, Beatrice (2018). General Chemistry: Atoms First. Boston: Cengage Learning. p. 109. ISBN 978-1-337-61229-6. {{cite book}}: |access-date= requires |url= (help)
  5. ↑ Young, Susan M.; Vining, William J.; Day, Roberta; Botch, Beatrice (2018). General Chemistry: Atoms First. Boston: Cengage Learning. p. 41. ISBN 978-1-337-61229-6. {{cite book}}: |access-date= requires |url= (help)
  6. ↑ Mackay, K. M.; Mackay, R. A.; Henderson, W. (2002). Introduction to Modern Inorganic Chemistry (6th ed.). Cheltenham: Nelson Thornes. section 2.13: Ionization potential. ISBN 0-7487-6420-8. Retrieved 2026-10-23.
  7. ↑ Young, Susan M.; Vining, William J.; Day, Roberta; Botch, Beatrice (2018). General Chemistry: Atoms First. Boston: Cengage Learning. pp. 106–107. ISBN 978-1-337-61229-6. {{cite book}}: |access-date= requires |url= (help)
  8. ↑ Young, Susan M.; Vining, William J.; Day, Roberta; Botch, Beatrice (2018). General Chemistry: Atoms First. Boston: Cengage Learning. pp. 110–111. ISBN 978-1-337-61229-6. {{cite book}}: |access-date= requires |url= (help)
  9. ↑ 9.0 9.1 9.2 9.3 Mackay, K. M.; Mackay, R. A.; Henderson, W. (2002). Introduction to Modern Inorganic Chemistry (6th ed.). Cheltenham: Nelson Thornes. section 5.1: The formation of ionic compounds. ISBN 0-7487-6420-8. Retrieved 2026-10-23.
  10. ↑ Brock, William H. (1992). The Fontana History of Chemistry (ePub ed.). London: Fontana Press. Chapter 10: On the Dissociation of Substances Dissolved in Water, section Electrochemistry from Faraday to Arrhenius. ISBN 9780007394968. {{cite book}}: |access-date= requires |url= (help)
  11. ↑ Brock, William H. (1992). The Fontana History of Chemistry (ePub ed.). London: Fontana Press. Chapter 10: On the Dissociation of Substances Dissolved in Water, section The Ionic Theory. ISBN 9780007394968. {{cite book}}: |access-date= requires |url= (help)
  12. ↑ Brock, William H. (1992). The Fontana History of Chemistry (ePub ed.). London: Fontana Press. Chapter 10: On the Dissociation of Substances Dissolved in Water, section The Reception of the Ionic Theory / The Ionic Theory. ISBN 9780007394968. {{cite book}}: |access-date= requires |url= (help)
Retrieved from "https://tilcayo.cat/w/index.php?title=Ion&oldid=175279"