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

Sodium

Potassium

Rubidium

Caesium

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This question was previously asked in

UPSC NDA-1 – 2018

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The alkali metals are elements in Group 1 of the periodic table: Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Caesium (Cs), and Francium (Fr). As we move down the group, the atomic size increases. The metallic bond strength weakens with increasing atomic size because the valence electron is further from the nucleus and shielded by more inner electron shells, leading to lower melting and boiling points. Therefore, among the given options (Sodium, Potassium, Rubidium, Caesium), Caesium (Cs) has the largest atomic size and thus the weakest metallic bonding, resulting in the lowest melting point.

Melting points of alkali metals decrease down the group due to decreasing metallic bond strength as atomic size increases.

The melting points of the given alkali metals are approximately: Sodium (97.8 °C), Potassium (63.5 °C), Rubidium (39.3 °C), and Caesium (28.5 °C). Caesium’s melting point is just above room temperature.

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