Q.Which of the alkali metal is having least melting point ?

Group 1 elements (Li, Na, K, Rb, Cs, Fr) all have a single electron in the outermost shell, electronic configuration $ns^1$, so they lose this electron very easily (lowest ionisation enthalpies of their periods) to form $M^+$ ions and act as strong reducing agents. They are soft metals that can be cut with a knife, have low density (Li, Na and K float on water), and are the largest atoms/ions in their respective periods. Melting and boiling points are low and fall steadily down the group (Li highest, Cs lowest) because metallic bonding weakens as the atoms get bigger and the delocalised valence electron is held more loosely. Because ionisation enthalpy keeps falling down the group, chemical reactivity increases from Li to Cs, so heavier alkali metals react almost explosively with water, air and halogens. Owing to this …