According to the law of conservation of matter, the number of atoms of each element in the reactants must equal the number of atoms of that element in the products.
In this case, you have:
- 20 atoms of hydrogen (H) reacting to produce 20 molecules of hydrochloric acid (HCl).
- Each molecule of HCl contains 1 hydrogen atom and 1 chlorine atom.
Since there are 20 molecules of HCl produced, there must be 20 atoms of chlorine (Cl) involved in the reaction (one chlorine atom for each molecule of HCl).
Therefore, the number of chlorine atoms that reacted with the hydrogen is 20 atoms of chlorine.