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Chemistry LibreTexts

Sizes of Ions

Sizes of ions influence:

  • packing of ions in ionic lattices, and therefore, the lattice energy
  • biological recognition - some ions can pass through certain membrane channels, others may be too large

The size of an ion is influenced by:

  • nuclear charge
  • number of electrons
  • valence orbitals


  • formed by removing one or more valence electrons
  • vacates the most spatially extended orbitals
  • decreases the total electron-electron repulsion in the outer orbital

Cations are therefore smaller than the parent atom


  • formed by addition of one or more valence electrons
  • fills in the most spacially extended orbitals
  • increases electron-electron repulsion in outer orbital

Anions are therefore larger than the parent atom

For ions of the same charge (e.g. in the same group) the size increases as we go down a group in the periodic table

As the principle quantum increases the size of both the parent atom and the ion will increase

How does the nuclear charge affect ion size?

Consider the following collection of ions:

ion electrons protons
O2- 10 8
F- 10 9
Na+ 10 11
Mg2+ 10 12
Al3+ 10 13

Each ion:

  • contains the same number of electrons (10; with configuration 1s22s22p6) and are thus termed a collection of isoelectronic ions
  • varies in the nuclear charge

The radius of each ion decreases with an increase in nuclear charge:

Oxygen and fluorine precede neon and are nonmetals, sodium, magnesium and aluminum come after neon and are metals.