Octet rule exceptions and examples

Ions and the octet rule

The octet rule provides a way to predict what kind of ion an atom will form. The rule usually applies to the representative elements. The rule has many exceptions, but is still very useful. The breaks down for elements in periods 3 through 7.

The Octet Rule:

Main-group elements are more stable when they have a complete outer shell of 8 valence electrons.In other words, the main-group elements tend to react so they have eight valence electrons.

How many valence electrons are there in a chloride anion?click for answer

Example:

What kind of ion do you expect to be formed by calcium?

Answer:

Calcium will form a Ca²⁺ cation. This is based on the fact that calcium is in Group 2A with two valence electrons.

The electron configuration for the 20 electrons in calcium is 1s²2s²2p⁶3s²3p⁶4s².

This can be written in a "shorthand" notation as [Ar]4s². Here the 18 inner electrons in the 1s²2s²2p⁶3s²3p⁶ are represented by the symbol [Ar]. The outer electrons are in the 4s subshell.

The neutral atom is converted to the +2 ion when the outer electrons are removed. The octet rule predicts an atom will form an ion with a rare gas electron configuration. The rare gas in this case is the rare gas from the previous period.

Particle

Electron configuration

Shorthand configuration core electrons are indicated by rare gas symbol

Ca atom

1s²2s²2p⁶3s²3p⁶4s²

[Ar]4s²

Ca²⁺ cation

1s²2s²2p⁶3s²3p⁶

[Ar]

Note the shorthand notation shows the rare gas core and the subshells for the outer electrons.