Q.i. Why cations are Lewis acids ?
Step 1. The Lewis theory (section 3.3.3) defines an acid as any species that accepts a share in an electron pair.
Step 2. A cation carries a net positive charge and, in many cases, has empty (vacant) orbitals available -- both features make it electron-pair-deficient relative to a neutral or negatively charged species.
Step 3. This electron deficiency means a cation is drawn to, and can accept, a lone pair of electrons offered by an electron-rich donor species (a Lewis base) -- for example, a metal cation accepting lone pairs from surrounding water molecules to form an aquo-complex, exactly as illustrated by the example in this chapter's exercise Question 1 (item vi).
Step 4. Because 'accepting a shared electron pair' is precisely the defining behaviour of a Lewis acid, and cations do exactly this, cations are classified as Lewis acids.
Cations are Lewis acids because their positive charge (and often vacant orbitals) let them accept a shared electron pair from an electron-pair-donating species.
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