How do you balance $\text{Mg + HCl}\to {\text{MgCl}}_{\text{2}}{\text{ + H}}_{\text{2}}$?

Maintaining equal molecularity for each atom present on the reactant side as well as the product side of the specific chemical reaction is necessary for balancing a chemical equation.

In light of this, magnesium interacts with hydrochloric acid to produce magnesium chloride and hydrogen gas, as indicated in the following chemical reaction:

$\mathrm{Mg}+\mathrm{HCl}\to {\mathrm{MgCl}}_2+{\mathrm{H}}_2$

The following factors must be kept in mind in order to balance the chemical equation provided above: - First, we count the number of each atom on both the left and right sides of the equation.
Therefore, it is true that there is just one magnesium (Mg) atom present on both the left hand side of the chemical reaction, known as the reactant side, and the right hand side, known as the product side.
- One chlorine atom (Cl) is present on the left, while two chlorine atoms are present on the right, which is incorrect.
Similarly, it is incorrect to have only one hydrogen atom (H) on the left side and two hydrogen atoms (H) on the right side.

So, in order to ensure that each atom has the same molecularity on both the reactant side and the result of the reaction, we will balance the reaction described above in the way described below. A balanced equation is also shown as:

$\mathrm{Mg}+2\mathrm{HCl}\to {\mathrm{MgCl}}_2+{\mathrm{H}}_2$

Two hydrogen atoms (H), two chlorine atoms (Cl), and one magnesium atom (Mg) are present on the left and right sides of the chemical reaction, respectively, in the above balanced equation.

Hence, option A is correct.