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Oxidation State

Module by: Andrew R. Barron. E-mail the author

The oxidation state of an element is defined as the formal charge on the atom if all bonds were assumed to be fully ionic.

In an ionic compound the oxidation state is equal to the charge on the ion, e.g., in NaCl the charge on the sodium is +1 and the oxidation state is also +1. In contrast, the charge on an atom in a covalent compound never approaches the charge implied from the oxidation number, e.g., in CCl4, the oxidation state of carbon is +4 but the charge on the carbon atom is significantly less.

Thus, while oxidation state is a simple formulism it is very useful in a number of ways:

  1. 1) Classification of compounds of elements, especially those of the transition metals.
  2. 2) Enables the balance of reduction-oxidation (redox) reactions.

To determine the oxidation state of an element within a particular compound the following steps act as a guideline.

  1. Step 1) The oxidation state of any atom of any element in its elemental form is zero.
  2. Step 2) The oxidation state of a monatomic ion is equal to its charge.
  3. Step 3) Any homoleptic X-X bonds (e.g., the C-C bond in H3C-CH3) are assumed to be non-ionic and do not contribute to the oxidation state of X.
  4. Step 4) Fluorine always has oxidation state of –1.
  5. Step 5) Elements of Group 1 (IA) (except hydrogen) have an oxidation state of +1 in compounds.
  6. Step 6) Elements of Group 2 (IIA) have an oxidation state of +2 in compounds.
  7. Step 7) Elements of Group 17 (VIIA) have an oxidation state of -1 when they combine with elements below or to the left of their position in the periodic table.
  8. Step 8) Oxygen is usually assigned the oxidation state of –2, except in compounds with fluorine, oxygen has a positive oxidation number.
  9. Step 9) Hydrogen is assigned the oxidation state of +1 with non-metals and –1 with metals.
  10. Step 10) In any compound the sum of the oxidation states (oxidation numbers) is equal to the overall charge.

Exercise 1

What is the oxidation state of sulfur in Na2SO4?

Solution:

The two sodium atoms each have an oxidation state of +1, while the oxygen atoms have an oxidation state of –2, and the overall charge is 0.

  1. Step 1. overall charge = sum of oxidation states
  2. Step 2. 0 = (2 x oxidation state of Na) + (oxidation state of S) + (4 x oxidation state of O)
  3. Step 3. 0 = (2 x +1) + (oxidation state of S) + (4 x -2)
  4. Step 4. oxidation state of S = 0 – (2 x +1) - (4 x –2)
  5. Step 5. oxidation state of S = +6

Exercise 2

What is the oxidation state of sulfur in Na2SO3?

Solution

The two sodium atoms each have an oxidation state of +1, while the oxygen atoms have an oxidation state of –2, and the overall charge is 0.

  • overall charge = sum of oxidation states
  • 0 = (2 x oxidation state of Na) + (oxidation state of S) + (3 x oxidation state of O)
  • 0 = (2 x +1) + (oxidation state of S) + (3 x -2)
  • oxidation state of S = 0 – (2 x +1) - (3 x –2)
  • oxidation state of S = +4

Exercise 3

What is the oxidation state of sulfur in H2S?

Solution

The two hydrogen atoms each have an oxidation state of +1 and the overall charge is 0.

  • overall charge = sum of oxidation states
  • 0 = (2 x oxidation state of H) + (oxidation state of S)
  • 0 = (2 x +1) + (oxidation state of S)
  • oxidation state of S = 0 – (2 x +1)
  • oxidation state of S = -2

In writing the oxidation state of an element within a compound it is common to use a Roman numeral, rather than the charge, i.e., Al(III) rather than Al3+.

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A lens is a custom view of the content in the repository. You can think of it as a fancy kind of list that will let you see content through the eyes of organizations and people you trust.

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What are tags? tag icon

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