The chemical activity series is the arrangement of the elements descendingly according to their chemical activity for reactions like simple substitution reactions and redox reactions. Elements above hydrogen can replace hydrogen from water and acids as they are more active than hydrogen. Since cesium is more active than hydrogen, it replaces it in hydrochloric acid.
2 cesium atoms loses 2 electrons first;
2Cs --> 2Cs+ + 2e-
2 hydrogen atoms recieve the two electrons;
2H+ + 2e- --> H2
The net ionic equation is;
2Cs + 2HCl --> CsCl + H2