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

In the periodic table, the melting points of alkali metals (Group 1) decrease as you move down the group [1]. This trend occurs because the atomic radius increases, leading to a decrease in the strength of the metallic bonds between the atoms. Among the options provided, Lithium has the highest melting point at approximately 180°C, followed by Sodium (98°C), Potassium (63°C), and Rubidium (39°C). Caesium (Cs) has the lowest melting point of the four, recorded at approximately 28.5°C [1]. This temperature is so low that caesium can melt if held in the palm of a hand [1]. While Francium (Fr) is predicted to have an even lower melting point (around 22°C), it is not among the provided options [1]. Therefore, Caesium is the correct answer.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 3: Metals and Non-metals > Table 3.1 > p. 40