Alkali Metal Reactivity Increases Down the Group
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Group 1 alkali metal reactivity increases down the group because atomic radius grows and ionization energy falls, making the single valence electron progressively easier to remove.
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Cesium reacts more violently with water than sodium does, and sodium more violently than lithium.
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Reactivity here means ease of oxidation, not physical hardness — the alkali metals actually grow softer, not tougher, down the group.