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Oxidation and reduction

Oxidation occurs when electrons are lost by an atom or a molecule.

A zinc ($$\ce{Zn}$$) atom is oxidised when it loses two electrons to become a soluble zinc ion ($$\ce{Zn^2+}$$).

In covalent bonding, oxidation occurs when an atom gets less than an even share of the electrons it shares in a bond.

The carbon in methane ($$\ce{CH4}$$) is oxidised when methane reacts with oxygen to form carbon dioxide ($$\ce{CO2}$$).

The oxygen atoms exert a strong pull on the electrons in the bonds with carbon in $$\ce{CO2}$$ so that the electrons are more "on the side of the oxygen than the side of the carbon. The carbon atom gets less than an even share of the electrons it shares with the oxygen atoms. Carbon has thus "lost" electrons and is oxidised.

Oxygen atoms exert strong pulls on electrons within molecules. Atoms are thus usually oxidised when they form bonds with oxygen.

This is how the term oxidation originally got its name.

Reduction occurs when electrons are gained by an atom, molecule or ion.

A copper ion ($$\ce{Cu^2+}$$) is reduced when it acquires two electrons to become a copper atom ($$\ce{Cu}$$).

In covalent bonding, an atom is reduced when it gets more than an even share of the electrons it shares in a bond.

Hydrogen atoms exert a weak pull on electrons within molecules. Molecules are thus reduced when they form more bonds with hydrogen.

The carbon atoms in ethene ($$\ce{C2H4}$$) are reduced when ethene reacts with hydrogen gas to form ethane ($$\ce{C2H6}$$).

The carbon atoms get more than an even share of electrons in the bonds. The carbon atoms have thus "gained" electrons and are reduced.

Copper ions in blue copper (II) sulfate are reduced to form copper atoms (which are solid). The blue colour fades as solid copper is deposited.
Copper ions in blue copper (II) sulfate are reduced to form copper atoms (which are solid). The blue colour fades as solid copper is deposited.