What are the differences between oxidation numbers and ionic charge? Do I have to write e.g #Al^(3+)# or #Al^(+3)#?

1 Answer
Jun 29, 2015

Here's how to think about charge vs. oxidation number.


An atom is said to have a nonzero net charge only when it either loses or gains electrons to become an ion.

If an atom loses electrons, it develops a positive charge and becomes a cation. Likewise, if an atoms gains electrons, it develops a negative charge and becomes an anion.

Charges can only exist on ions.

But what if an atom shares its electrons?

If an atom is part of a covalent bond, then you can't really sy that it has a net charge because it neither loses, nor gains electrons - it merely shares them.

This is where oxidation numbers come into play. You can assign a "fictional charge" to an atom that's part of a covalent bond by assuming that it either takes the bonding electrons from the other atom completely, or loses them completely.

In other words, you assume that the atom behaves as an ion.

You determine which atom "takes" the electrons and which atom "loses" the electrons by looking at the electronegativity of the two atoms.

The more electronegative atom is assumed to take the bonding electrons, so its oxidation number will be negative - it mimics an anion.

The less electronegative atom is assumed to lose the bonding electrons, so its oxidation number will be positive - it mimics a cation.

So, when it comes to ionic compounds, their ions have actual charges. When you're dealing with actual charges, the notation is always #"3+"#, #"2+"#, #"2-"#, #"3-"#, and so on.

When you're dealing with a covalent compound, their atoms have oxidation numbers. In such cases, the oxidation states of the "fictional ions" will always be written #"+1"#, #"+2"#, #"-2"#, #"-3"#, and so on.

So, bottom line

  • Ionic charge #-># #"3+"#, #"3-"#
  • Oxidation number #-># #"+3"#, #"-3"#