* The number of atoms of each element on the reactants' side (left side) of the equation is equal to the number of atoms of that same element on the products' side (right side) of the equation.

In simpler terms:

* You have the same number of each type of atom before and after the chemical reaction.

This is based on the Law of Conservation of Mass, which states that mass cannot be created or destroyed in ordinary chemical reactions.

For example:

Unbalanced equation:

H₂ + O₂ → H₂O

Balanced equation:

2H₂ + O₂ → 2H₂O

In this example, we have:

* Reactants: 2 hydrogen atoms (H₂) and 2 oxygen atoms (O₂)

* Products: 4 hydrogen atoms (2H₂) and 2 oxygen atoms (2O₂)

The balanced equation now has the same number of each type of atom on both sides.