Which one of the following alkali metals has lowest melting point ?

The correct answer is D, Caesium. Caesium melts at about 28.5 °C, the lowest of these alkali metals, so it can turn liquid on a warm day or in the hand.

The melting points of the alkali metals fall steadily down group 1: lithium about 181 °C, sodium about 98 °C, potassium about 63 °C, rubidium about 39 °C and caesium about 28.5 °C. As atoms grow larger down the group, the single outer electron is farther from the nucleus, so metallic bonding between atoms becomes weaker and less heat is needed to melt the metal. Caesium is also the basis of atomic clocks, which define the second.

A is wrong because sodium melts at about 98 °C, much higher than caesium. B is wrong because potassium melts at about 63 °C. C is wrong because rubidium, at about 39 °C, is close but still higher than caesium.

Exam tip: alkali metal melting points fall from Li to Cs; caesium and mercury are among the few metals liquid near room temperature.