Neutral atoms of each element contain an equal number of protons and electrons. The number of protons determines an element's atomic number and is used to distinguish one element from another. Together, the number of protons and the number of neutrons determine an element's mass number.
<u>Answer:</u> The atoms of every element on both the sides of the reaction must be same.
<u>Explanation:</u>
We are given a chemical equation and we need to balance it. Every equation follows Law of Conservation of mass.
This law states that in a chemical reaction, the number of atoms of each element must be same on both the sides of the equation.
For the given chemical equation:

On reactant side:
Number of zinc atoms = 1
Number of hydrogen atoms = 1
Number of chlorine atoms = 1
On product side:
Number of zinc atoms = 1
Number of hydrogen atoms = 2
Number of chlorine atoms = 2
As, the number of hydrogen and chlorine atoms on reactant and product side is not same, so we need to add a stoichiometric coefficient in-front of HCl on the reactant side. The balanced chemical equation becomes:
