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What does an oxidation number actually mean

An oxidation number is a bookkeeping value that represents the hypothetical charge an atom would have if all bonds to atoms of different elements were treated as completely ionic. It lets you track electron transfer in redox reactions by assigning each element a formal charge that sums to the molecule’s overall charge.

Chemistry · Redox reactions

Oxidation numbers are not measured properties; they are assigned to help balance redox equations. For example, in NaCl the Na atom is given +1 because it loses one electron to Cl, which is assigned –1. The sum (+1)+(-1)=0 matches the neutral compound.

How oxidation numbers are assigned

Key rules for assigning oxidation numbers

  • The oxidation number of a free element is 0.
  • For a monatomic ion, the oxidation number equals the ion’s charge.
  • Hydrogen is usually +1 (except in metal hydrides where it is –1).
  • Oxygen is usually –2 (except in peroxides, superoxides, or when bonded to fluorine).
  • The sum of oxidation numbers in a neutral molecule is 0; in an ion it equals the ion’s charge.

Using oxidation numbers to identify oxidation and reduction

  1. 1Write the formula and assign oxidation numbers to each atom using the rules.
  2. 2Identify atoms whose oxidation numbers increase; they are oxidized (lose electrons).
  3. 3Identify atoms whose oxidation numbers decrease; they are reduced (gain electrons).
  4. 4Balance the electron transfer by ensuring total electrons lost equal total electrons gained.

Common oxidation numbers for selected elements

ElementTypical oxidation number
Lithium+1
Carbon–4, –3, –2, –1, 0, +1, +2, +3, +4
Oxygen–2
Chlorine–1, +1, +3, +5, +7
Sulfur–2, +4, +6

Check yourself

If the oxidation number of chlorine in ClO3−\mathrm{ClO_3-} is +5, how many electrons are gained per chlorine atom when it is reduced to Cl−\mathrm{Cl-}?

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