On moving down the group in alkali metals, the melting point generally decreases. 

As you move down the alkali metal group (Group 1 of the periodic table), the atomic size or atomic radius increases. This increase in atomic size leads to a decrease in the strength of metallic bonds between the alkali metal atoms. Weaker metallic bonding results in lower melting points because less energy is required to overcome these weaker bonds and convert the solid metal into a liquid state.

$$\begin{gathered} \mathrm{Element} \mathrm{M}.\mathrm{P}. \left(\mathrm{K}\right) \\ \mathrm{Na} 371 \\ \mathrm{K} 336 \\ \mathrm{Rb} 312 \\ \mathrm{Cs} 302 \end{gathered}$$