Oxidation States of Ions and Atoms


Oxidation states are a measure of the difference in the number of protons and electrons in an ion or atom

Oxidation states indicate the total number of electrons that have been removed or gained by an atom in the formation of an ion. Metals lose electrons forming positive ions and this is known as oxidation. In the equation below you can see the transition metal vanadium losing two electrons and becoming a 2+ charged ion.

As this is a transition metal very often when writing the name of a transition metal compound roman numerals are used to indicate the charge on metal ion. For example the compound vanadium oxide with the formula VO2 would be written as vanadium (IV) oxide indicating that the vanadium ion has a +4 oxidation state.

Non metals gain electrons in a process known as reduction. Reduction often sees the lowering of an oxidation state and is negative as a value. For example in the equation below we can see the non metal sulphur gaining 2 electrons to become a negatively charged sulphur ion.

Some general rules when assigning oxidation states are below:

  1. Hydrogen ions H+ always have an oxidation state of +1
  2. Oxygen has an oxidation state of -2 except for when it is in a peroxide where it is -1
  3. Group 1 metals have an oxidation state of +1 and Group 2 have an oxidation state of +2.
  4. Fluorine always has the oxidation state of -1
  5. Chlorine is always -1 except when it is in a compound with oxygen or fluorine.
  6. Elements that are not in compounds or are diatomic have an oxidation state of 0.
  7. Polyatomic ions can have an overall positive and negative charge

A good example is the polyatomic ion sulphate containing sulphur and oxygen with the formula SO4⁴2-. Overall you can see the ion has a negative -2 oxidation state. The oxygen atoms will have a -2 oxidation state of which there are 4 of them for an overall -8 state but sulphur must have an oxidation state of +6 as the overall charge on the ion is -2.

Terms in section
Oxidation state

Oxidation state is the term used to describe the loss or gain of electrons of an element to produce a charged ion


The electron is the smallest sub atomic particle that make up the atom. Has a negative charge and is located in shells that orbit the nucleus

Transition metals

Transition metals are the central section of the periodic table containing the majority of the metals. Also have d sub orbitals producing certain chemical properties


Oxidation is the term given to the process when an atom loses an electron to become a positively charged ion


Reduction is the term given to the process when an atom gains an electron to become a negatively charged on


Metals are located on the left hand side of the periodic table and comprising a large part of the periodic table. Metal elements have metallic bonding where positive ions are held in sea of delocalized electrons.

Non Polar

A non polar molecules is a molecule that has an electronegativity under 0.5 meaning the electrons are evenly shared between the two atoms


Ionisation Energies


Radioactivity and the Decay of Nuclei