Sizes of Ions

Basic Concepts of Chemical Bonding

Sizes of Ions

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

Cations

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

Anions

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
O^2-	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.

1996 Michael Blaber