Predicting Stable Anions and Cations

Main group elements (Group A) will gain or lose minimum number of electrons to have a filled shell (noble gas configuration).

Example:

Na 1s² 2s² 2p⁶ 3s¹ → Na⁺ 1s² 2s² 2p⁶ + e^-

Cl 1s² 2s² 2p⁶ 3s² 3p⁵ + e^- → Cl^- 1s² 2s² 2p⁶ 3s² 3p⁶

Transition group elements (Group B) generally lose outer shell s - electrons. Many also lose electrons to have either a filled or half-filled subshell.

Example:

Cu [Ar] 4s² 3d⁹ → Cu²⁺ [Ar] 3d⁹ + 2 e^-

The Cu²⁺ lost its outer shell s electrons.

Fe [Ar] 4s² 3d⁶ → Fe³⁺ [Ar] 3d⁵ + 3 e^-

The Fe³⁺ has a half-filled 3d subshell.

Ionic sizes

A cation is smaller than its parent atom. An anion is larger than its parent atom.

Generally, ion size increases going down a group. Change in ion size horizontally is complicated because we change from cations on the left to anions on the right. However, it is helpful to look at the relative size of isoelectronic ions.

Isoelectronic Ions-- ions containing the same number of electrons.

For example, O^2-, F^-, Na⁺, Mg²⁺, and Al³⁺ all have the electron configuration of Neon. But the charge of the nucleus increases from +8 on Oxygen to +13 for Aluminum. So the same number of electrons is Aluminum (Al³⁺) will be bound much closer to the nucleus than the electrons of O^2-.

For a series of isoelectronic ions, the size decreases as the nuclear charge, z, increases.

Big Picture: Chemistry is all about transferring electrons between different atoms and/or molecules. The different chemical properties of each atom in the periodic table is a result of the different degrees of which each atom wants to gain or lose electrons.