Oxidizing reducing characteristics of elements and compounds

Chemical properties of elements vary with according to the oxidation number. Oxidation number can be positive or negative of atoms.


Examples

  • When oxidation number increases, acidid strength increases.
  • When metal has an higher oxidation number, it can hydrolysis.

What is oxidation?

If an atom removes electrons completely or partially, it is defined as oxidation. When an atom looses its electrons, it is oxidized and oxidation number will be positive. Metals prefer to remove electrons. Therefore oxidation number of metals are always positive.

  • Sodium remove one electron to form Na+ ion. Therefore we say, Na is oxidized.
  • Like Na, all other alkali metals only oxidized.

What is reduction?

If an atom takes electrons completely or partially towards itself is called as reduction. When an atom gains electrons, it is reduced and oxidation number will be negative.

  • A Cl2 molecule takes two electrons from outside to form two Cl- ions. In that occasion we say Cl2 is reduced.

Note that, oxidation and reduction reactions should occur at same time. Only oxidation or reduction cannot be occurred alone.



Oxidation number

The number of electrons taken (completely or partially) from outside or number of electrons removed (completely or partially) is defined as oxidation number.


  • If electrons are removed, that atom has less number of electrons than number of protons. Thus oxidation number become positive.

If one electron is removed, oxidation number is +1

If two electrons are removed, oxidation number is +2.

positive oxidation numbers

Alkali metals forms only +1 oxidation number and alkaline earth metals forms only +2 oxidation number.


  • If electrons are taken, that atom has higher number of electrons than number of protons. Thus oxidation number become negative.

If one electron is taken, oxidation number is -1

If two electrons are taken, oxidation number is -2.

negative oxidation numbers

Covalent bonds

Atoms attract electrons in the bonds. But some atoms (F, O, N, Cl) can attract electrons more than others. Electronegativity values tells us about those attraction ability of atoms. Here are some examples.


Consider hydroxyl group

Electronegativity values of O and H are 3.5 and 2.1 respectively. Oxygen's electronegativity is much larger than H. Therefore electrons in the O-H bond are attracted towards O atom.

So, Hydrogen partially looses its electron. Therefore H gets +1 oxidation state.

Oxygen partially takes the electron of H. Therefore O gets -1 oxidation state from that part. Also O has taken an electron from outside because of that O gets another -1 oxidation value. So complete oxidation number ox oxygen is -2.

hydroxyl group oxidation numbers

How to calculate

Ionization energy decreases along a group. It means removing/eliminating an electron gets easier along a group. When eliminating an electron gets easier, ability of being oxidized increases. Otherwise we can say, reducing characteristics increase along a group.
Ex: reducing characteristic of K is higher than Na.

Ionization energy increases along a period (left to right), therefore removing/eliminating an electron is getting hard. So oxidizing ability decreases, but reducing ability increases. Otherwise we can say, oxidizing characteristic increases along a period (left to right).

Related Tutorials to Oxidizing - Reducing Characteristics