Lithium, sodium, potassium, rubidium, and cesium are all part of a group called the alkali metals. They are all found in group 1 of the periodic table. That is to say that they are all located in the first vertical line on the left side of the periodic table. They are all soft metals in elemental form and they all react violently with water. They react so strongly because they all readily lose an electron to form the +1 cation very easily. This is because they only contain one electron in their valence shells so it is easily removed to achieve a stable octet of electrons in the outermost energy shell.
The all have ium at the end of their name. Just kidding. All elements listed above are part of the alkali metal family (groups 1) and are highly reactive in water due to having only 1 valence electron which they are ready to lose in order to gain a pseudo noble gas configuration.