Answer is: obey the Law of conservation of mass.
For example, balanced chemical reaction:
Pb(NO₃)₂(aq) + Li₂SO₄(aq) → PbSO₄(s) + 2LiNO₃(aq).
According to principle of mass conservation, number of atoms must be equal on both side of balanced chemical reaction.
There are two lithium atoms, one lead atom, one sulfur atom, ten oxygen atoms and two nitrogen atoms on both side of reaction.
