What is an Ion?
Ions are identified by the use of a superscript to show the sign and size of their electric charge.
Anions and Cations
Negatively charged ions are called anions, and positively charged ions are called cations.
Examples of Anions
Examples of Cations
Ions form when doing so minimizes the total potential energy of the chemical species involved in the chemical reaction. Often, this is achieved by allowing atoms of different elements to achieve a full shell of electrons.
Consider the salt lithium fluoride:
Lithium atoms have 3 electrons, meaning they have electrons in two shells: 2 in the first shell and 1 in the second. By losing an electron to become a cation, lithium gets a stable electron arrangement, identical to that of the noble gas helium.
Fluorine atoms have 9 electrons, meaning they have 2 in the first shell and 7 in the second. By gaining an electron to become an anion the fluoride ion gets a stable electron arrangement, identical to that of the noble gas neon.
And so the redox reaction of lithium with fluorine to form an ionic compound is energetically favorable:
Cations are smaller than the original atom - consider how the Lithium atom has electrons in two shells, while the lithium ion has electrons in only one shell. With more electrons, anions are larger than the original atom.
Ion sizes are measured by the ionic radius.
Ionic Radii in Picometers
The gray color in the diagrams shows the size of the original atom. The blue (cation) and yellow (anion) shows the size of the ion. (Diagrams by Chris King.)
Ionic compounds, such as sodium chloride (NaCl) are formed because of the electrostatic attraction between anions and cations. Ionic compounds are electrically neutral, because the total number of positive charges and the total number of negative charges are always equal.
Considering the chemical elements, the highest tendency to form ionic compounds exists in the least electronegative metals and the most electronegative non-metals.
In general, solid ionic compounds do not carry electric current, because the ions are locked in fixed positions in a crystal lattice. Ionic compounds only conduct electricity when molten or when dissolved in solution, when the ions are released from the crystal lattice and become mobile.
The diagram of atomic radii in picometers was created by Popnose (Chris King) and is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.