Oxidation Numbers (IB Chem HL 1 Unit 6)

Oxidation State

• a number assigned to an atom to show the number of electrons transferred in forming a bond

• based entirely on the ionic model of bonding (it is the charge that the atom would have if the compound was theoretically completely composed of ions)

• Oxidation states are realistically a measure of how electrons are distributed within the representative particle

Position of + or - symbol matters

Charges: 2+

Oxidation states: +2

Oxidation is…

• The addition of oxygen

• The removal of hydrogen

• Any electron loss

• An increase in the oxidation state

Types of Oxidation states (Elements)

• Elements are always zero

  • because the atoms have no charge and the electrons are distributed equally between the atoms

Types of Oxidation states (Binary Ionic Compounds)

• Binary Ionic Compounds are the same as the respective charges

  • Because the charges represent the distribution of electrons corresponding to the number of electrons transferred

Types of Oxidation states (Covalent Compounds)

• The electrons distribution depends on the polarity of the bond and the relative electronegativities of the bonding atoms

• The more electropositive atom will lose control of some of its electrons and have a positive oxidation state

• The more electronegative will gain control of more electrons and have negative oxidation state.

Oxidation State Rules

• Atoms in the free (uncombined) elements have an oxidation state of zero

• In simple ions, the oxidation state is the same as the charge of the ion

• The oxidation states of all atoms in a neutral (uncharged) compound must add up to zero

• The oxidation states of all atoms in a polyatomic ion must add up to the charge of the ion

• The oxidation state for an element in a compound is the same as the most common charge (Group 1 metals +1 etc.)

• Fluorine has the oxidation state of -1 in all Compounds since it is always the more electronegative element

• Oxygen usually has the oxidation state of -2

  • In peroxides, its oxidation state is -1

  • when bonded with fluorine it is electropositive (+2)

• Chlorine usually has the oxidation state of -1

  • When bonded with fluorine or oxygen, it is electropositive

• Hydrogen usually has the oxidation state of +1

  • When bondied with group 1 and 2 metals, forming ionic hydrides, it is more electronegative so it becomes -1

• The oxidation state of a transition metal in a complex ion can be worked out from the charge on the ligand

  • Common ligands: H₂O, NH₃, Cl^-, CN^-, OH^-

Oxidation State Format

The oxidation number is written as a Roman numeral