Ionic size (for the same ion) also increases with increasing coordination number, and an ion in a high-spin state will be larger than the same ion in a low-spin state. As with other types of atomic radius, ionic radii increase on descending a group. Nevertheless, ionic radius values are sufficiently transferable to allow periodic trends to be recognised. The ionic radius is not a fixed property of a given ion, but varies with coordination number, spin state and other parameters. The latter values are thought to be a more accurate approximation to the 'true' relative sizes of anions and cations in ionic crystals.

Some sources have retained Pauling's reference of r ion(O 2−) = 140 pm, while other sources prefer to list 'effective' ionic radii based on r ion(O 2−) = 126 pm. Ī major review of crystallographic data led to the publication of a revised set of ionic radii in 1976, and these are preferred to Pauling's original values. By comparing many different compounds, and with a certain amount of chemical intuition, Pauling decided to assign a radius of 140 pm to the oxide ion O 2−, at which point he was able to calculate the radii of the other ions by subtraction. However, it is not apparent what proportion of this distance is due to the size of the sodium ion and what proportion is due to the size of the chloride ion. For example, it can be readily determined that each side of the unit cell of sodium chloride is 564.02 pm in length, and that this length is twice the distance between the centre of a sodium ion and the centre of a chloride ion:Ģ = 564.02 pm

X-ray crystallography can readily give the length of the side of the unit cell of a crystal, but it is much more difficult (in most cases impossible, even with more modern techniques) to distinguish a boundary between two ions. The concept of ionic radius was developed independently by Goldschmidt and Pauling in the 1920s to summarize the data being generated by the (then) new technique of X-ray crystallography: it is Pauling's approach which proved to be the more influential. It is measured in either picometres (pm) or Angstrom (Å), with 1 Å = 100 pm. The ionic radius, r ion, is a measure of the size of an ion in a crystal lattice.

Calcium atomic radius