hard acid

chemistry

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June 29, 2026

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In chemistry, a "hard acid" is a classification within the Lewis acid-base theory. Lewis acids are substances that can accept an electron pair. Hard acids are characterized by being small, highly charged, and having a low polarizability. They tend to react preferentially with "hard bases," which are small, highly electronegative, and have low polarizability. Examples of hard acids include the proton (H⁺), alkali metal ions (like Na⁺), and alkaline earth metal ions (like Ca²⁺). This classification helps predict the reactivity and stability of complexes formed between acids and bases.1

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acid–base reaction
Lewis acids
…“soft” categories; as a rule, hard acids react preferentially with hard bases and, similarly, soft acids react with soft bases. The terms hard and soft are chosen to suggest that the atomic structures associated with hard acids and bases are rigid and impenetrable, whereas those associated with soft acids and…
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…“soft” categories; as a rule, hard acids react preferentially with hard bases and, similarly, soft acids react with soft bases. The terms hard and soft are chosen to suggest that the atomic structures associated with hard acids and bases are rigid and impenetrable, whereas those associated with soft acids and…
Read More