Question

Which should give the least vigorous reaction when dropped in water? Rb, Na, K, Cs, or Li

Answer 1

Given are the alkali metals from group-1. As we go down the group from Li to Cs reactivity order increases as the atomic size increases. By this trend Cs being most down in the group and largest in atomic size, Cs will have the most vigorous reaction with water. However, the smallest atomic radius metal is Li in this group and hence it will give the least vigorous reaction when dropped in the water.

As the atomic size increases, the distance between last valence shell electron and nucleus increase, making it easier and easier to lose the electron. Thus as size increases reactivity increase. Lithium is the smallest and Cs is the larget atom in this group. Hence Li will react least vigorously with water.

The answer is Li.