The principle of redox is the transfer of electrons between two chemical species. It therefore depends on their oxidation states.
Oxidation state:
A chemical element has the oxidation state (or oxidation number):
n+ if it has lost n electrons
n- if it has gained n electrons

1) Oxidation is a loss of electron(s): the element’s oxidation state increases. The substance that loses electrons is called a reductant
e.g. ferrous iron Fe2+ is oxidized to ferric iron Fe3+ by losing one electron.

Fe2+ - e→ Fe3+

2) Reduction is a gain in electron(s): the element’s oxidation state decreases. The substance that gains electrons is called an oxidizer
e.g. mercuric mercury Hg2+ turns into mercurous mercury Hg+ in what corresponds to a reduction (gain of one electron)

Hg2+ + e→ Hg+

Generally speaking the relationship between oxidizers and reductants is as follows:

ox + ne- ↔ red
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