Magnesium reacts with oxygen to form magnesium oxide. Magnesium loses two electrons through oxidation to become Mg2+, becoming more stable by completing its octet. Oxygen gains these two electrons through reduction to become O2-, also completing its octet and achieving stability. The overall reaction sees magnesium oxidized by losing electrons while oxygen is reduced by gaining those electrons.

1. Reduction–Oxidation (REDOX) Reactions
In a chemical reaction electrons are lost or gained or shared.
When an element loses an electron or electrons the element is said to
have been oxidised. The element thereby becomes positively
charged.
element - e- → element +
When an element gains an electron or electrons the element is said
to have been reduced. The element thereby becomes negatively
charged.
element + e- → element -
Reduction = Gain of electrons (RedGain) Oxidation = Loss of electrons (OxiLoss)
Standard/ Class/ Grade XI Chemistry
Chapter 5 Redox Reactions
Gurudatta K Wagh, gkwagh@gmail.com

2. To understand the reduction-oxidation reaction let us take one
example. Magnesium and oxygen react to form magnesium oxide.
The balanced chemical equation of the reaction is
2Mg + O2 → 2MgO
How does the reaction proceed? We have to look at the electronic
configurations of both the reactants.
Magnesium has the electronic configuration 1s22s22p63s2
The third shell of magnesium contains 2 electrons. It requires six
more electrons to complete the octet. It is easier for magnesium to
become stable by losing the two electrons in the third shell.
Magnesium thus becomes stable since the second shell contains eight
electrons and the octet of electrons is completed. Thus we see
2Mg → 2Mg2+ + 4e-

3. The electronic configuration of oxygen is 1s22s22p4
Oxygen requires two electrons in its second shell to complete the
octet and become stable. It is easier for oxygen to accept two
electrons and achieve stability. Oxygen accepts the electrons released
by magnesium. We get
O2 + 4e- → 2O2-
The net reaction can be written as
2Mg + O2 + 4e- → 2Mg2+ + 4e- + 2O2-
Cancelling the 4e- on both the sides the reaction can be written as
2Mg + O2 → 2Mg2+ + 2O2- → 2MgO
The loss of electrons by magnesium is called oxidation of magnesium.
The gain of electrons by oxygen is called as reduction of oxygen.