Which one of the following elements is least reactive with water ?

Lithium

Sodium

Potassium

Cesium

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This question was previously asked in

UPSC NDA-1 – 2017

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Lithium, Sodium, Potassium, and Cesium are alkali metals belonging to Group 1 of the periodic table. The reactivity of alkali metals with water increases down the group due to the decreasing ionization energy and increasing atomic size, making it easier to lose the valence electron. Therefore, Lithium (Li) is the least reactive with water among the given options, while Cesium (Cs) is the most reactive.

Reactivity of alkali metals with water increases from Lithium to Cesium.

Lithium reacts slowly with water, producing hydrogen gas and lithium hydroxide. Sodium reacts vigorously, often melting due to the heat produced. Potassium, Rubidium, and Cesium react increasingly explosively with water.

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