Alkali metals show trends, such as electron affinity and melting point, that are not typically seen in other groups of metals. Moving from lithium to cesium, the melting point and the electron affinity becomes .

The melting point decreases and the electron affinity becomes more positive

Explanation:

As one moves down the alkali metal group, the melting point of elements decreases this results from the fact that the strength of the metallic bond in alkali metals decreases down the group with increasing cation size.

Alkali metals are electro positive. They donate rather than accept electrons. The positive values of electron affinity implies that they donate rather than accept electrons. Tremendous energy must be supplied if an alkali metal is to be made to accept an electron. Hence the positive value of electron affinity.

the answer for sodium is 0.186

the answer for rubidium is 0.251