A. properties which can be observed since it's what you're physically viewing
2 hydrogen and 2 chlorine on the reactant side(left of the arrow)
There is only 1 H and 1Cl on the products side so the balanced equation would be;
H2 + Cl2 —> 2 HCl
Each element shows the group and period they are through the number of energy level and valence electron in which can determine their place in the periodic table.
Searching "electron configuration of chromium on google images will easily provide you with the answer.
