The alkali metals have relatively low melting point. Which one of the

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The alkali metals have relatively low melting point. Which one of the following alkali metals is expected to have the highest melting point?

Li
Na
K
Rb
This question was previously asked in
UPSC NDA-2 – 2015
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The alkali metals are in Group 1 of the periodic table: Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Caesium (Cs), and Francium (Fr). Moving down the group, the metallic bonding strength generally decreases due to the increasing atomic size and shielding effect, which weakens the attraction between the positive ions and the delocalized valence electrons. Weaker metallic bonding leads to lower melting points. Therefore, the melting points of alkali metals decrease down the group. Among the options provided (Li, Na, K, Rb), Lithium is at the top of the group and is expected to have the highest melting point.
Melting points of alkali metals decrease down the group (from Li to Fr) due to decreasing metallic bond strength.
The approximate melting points are: Li (180.5 °C), Na (97.8 °C), K (63.5 °C), Rb (39.3 °C), Cs (28.5 °C). As expected, Lithium has the highest melting point among the given options. This trend is a typical example of how physical properties change across a group in the periodic table.

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