$$CaO(s) + 2HCl(aq) → CaCl_2(aq) + H_2O(l)$$

The number of atoms in the reactants are:

- CaO: 1 Ca atom and 1 O atom

- 2HCl: 2 H atoms, 2 Cl atoms

The number of atoms in the products are:

- \(CaCl_2\): 1 Ca atom, 2 Cl atoms

- \(H_2O\): 2 H atoms, 1 O atom

As you can see, the number of atoms of each element is the same on both sides of the equation. This means that the equation is balanced.