What is a limiting reactant?

The reactant that is finished itself during any chemical reaction is called the limiting reactant.

or,

The reactant that limits the amount of product is called the limiting reactant or limiting reagent.

If there is more than one reactant in a chemical reaction, it is not always possible to supply all the reactants in the required amounts when supplied after weighing. As a result, some reactants complete the reaction and none of it remains at the end while another reactant was also there in the reaction but some residue of it remains at the end. The reactant that finished itself during the reaction is called the limiting reactant. In a chemical reaction, the limiting reactant is determined by its amount. The amount of a limiting reactant will tell us how much of which reactant will react in the reaction, how much residue will be left and how much of which product will be produced.

4 oxygen molecules are put into 4 atoms of magnesium metal. Which is the limiting reactant here?

2Mg + O₂ → 2 MgO

The above equation tells us that, 1 O₂ molecule is necessary to react with 2 metallic atoms of Mg. So 2 O₂ molecules are necessary to completer the reaction with the given 4 Mg atoms. That’s mean 2 molecules of oxygen will be left over when all the given magnesium atoms will finish themselves in the reaction.

Here the magnesium atoms do not remain anymore at the end of the reaction. Therefore, the magnesium is the limiting reactant in this reaction.