One of the most important unifying concepts in chemistry is that
of electronegativity, and we have used it frequently. When two atoms
compete for the same electrons, their relative electronegativities
tell us which will win.
The end result can be described as an oxidation and a reduction.
When atoms lose electrons and become positive ions, they are oxidized.
The terminology comes from combination with oxygen, which for every
element except fluorine means a shift of electrons toward the more
electronegative oxygen atom. If fluorine were as common as oxygen
on our planet, we probably would call the loss of electrons fluoridation
instead of oxidation.
When electrons are given to a substance, the substance is reduced.
Again, the terminology is historical. It comes from the process
of reducing ores to pure metals, during which oxygen is removed
from the ore, and the metal ions are given electrons and converted
to neutral atoms.
The terms have become generalized beyond their original meanings,
and today any loss of electrons is called an oxidation,
and any gain of electrons is called a reduction, even if oxygen
is not involved. Since electrons are neither created nor destroyed
in chemical reactions, whenever one substance is oxidized, something
else must be reduced.
In the reaction of metallic sodium with chlorine gas, sodium is
oxidized and chlorine is reduced:
Na + 1/2Cl2
---------> Na+ + Cl-
An electron is transferred from sodium to chlorine