Oxidation States: Decoding Electron Behavior in Compounds
What is an Oxidation State?
Definition
Oxidation state
The oxidation state (or oxidation number) is a numerical value assigned to an atom in a compound. It reflects the hypothetical charge the atom would have if all bonds in the molecule were purely ionic.
- In simpler terms, it tells us how many electrons an atom has gained, lost, or shared during bonding.
- Oxidation states are vital for understanding redox reactions, where electrons are transferred between substances.
- They help us identify which species is oxidized (loses electrons) and which is reduced (gains electrons).
- In water (H₂O), oxygen is more electronegative than hydrogen, so it "pulls" the shared electrons closer to itself. As a result:
- Oxygen is assigned an oxidation state of −2 (it "gains" two electrons).
- Each hydrogen atom is assigned an oxidation state of +1 (it "loses" one electron).
Conventions for Assigning Oxidation States
- To assign oxidation states, chemists follow a systematic set of rules.
- These rules ensure consistency and accuracy across different compounds.
Rule 1: Free Elements
- The oxidation state of an atom in its elemental form is always 0.
- This applies to:
- Single atoms like Na, O₂, or Cl₂.
- Diatomic molecules like H₂ or N₂.
In O₂ (a molecule of elemental oxygen), each oxygen atom has an oxidation state of 0.
Rule 2: Sum of Oxidation States
The sum of the oxidation states of all atoms in:
- A neutral compound is 0.
- A polyatomic ion equals the charge of the ion.
- In H₂O, the sum is 0 : $2(+1) + (-2) = 0$.
- In the sulfate ion (SO₄²⁻), the sum equals −2: $ (+6) + 4(-2) = -2 $.
