The law of conservation of mass states that mass is neither created nor destroyed and in this equation you start with 1 molecule of Cl and 1 of H, but on the reactants side you end up with 2 of each. To make it follow the law of conservation of mass you would have to balance the equation
The vertical columns (groups) of the periodic table are arranged such that all its elements have the same number of valence electrons. All elements within a certain group thus share similar properties.
