---
title: "Acid-Base Models Beyond Bronsted: Lewis, Solvents and Hard and Soft"
description: "Protons are only one way to be an acid. Widening the definition to electron pair acceptance covers metal ions, boron compounds and non-aqueous solvents, and a further split into hard and soft explains"
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source: https://lightmysky.com/learn/science/acid-base-models-beyond-bronsted-lewis-solvents-and-hard-and-soft-mt_jarui-9yfx.md
retrieved: 2026-09-12
---

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# Acid-Base Models Beyond Bronsted: Lewis, Solvents and Hard and Soft

Protons are only one way to be an acid. Widening the definition to electron pair acceptance covers metal ions, boron compounds and non-aqueous solvents, and a further split into hard and soft explains which partners actually pair up.

Subject: Science · Area: Chemistry · Ages 19 to 20
Page: https://lightmysky.com/learn/science/acid-base-models-beyond-bronsted-lewis-solvents-and-hard-and-soft-mt_jarui-9yfx

## Ready when they can

- Identifies the Lewis acid and base in a reaction where no proton moves
- Classifies ions and molecules as hard or soft and predicts which pairs bind most strongly
- Explains an ore's mineral form using the hard and soft preference of its metal
- Applies the solvent-system definition to a named non-aqueous solvent

## Lesson: Acids without protons

A Lewis acid takes an electron pair and a Lewis base gives one. That single sentence covers reactions where no proton moves at all: boron trifluoride grabs ammonia by its nitrogen lone pair, so boron trifluoride is the acid and ammonia is the base. Metal ions count as acids too, since they accept pairs from the ligands around them.

Not every acid wants every base. Hard partners are small, highly charged, and hard to polarise, while soft partners are large and easily polarised. Hard prefers hard and soft prefers soft: fluoride and ammonia are hard bases, iodide and cyanide are soft ones, protons and aluminium ions are hard acids, and silver and platinum ions are soft.

**Example.** Ores show the rule written in stone. Magnesium is a hard acid, so you mine it as a carbonate, paired with a hard base. Mercury is a soft acid, so you mine it as a sulfide, paired with a soft base. Each mineral is simply the metal sitting beside its favourite partner.

**Tip.** Water is only one possible stage, and its acid base vocabulary fails the moment you leave it. The solvent system view lets each liquid define its own acid and base pair, which is why ammonia and sulfuric acid each run their own version of acid base chemistry with different lead actors.

**Recap.** Follow the electron pair to name the acid, match hard with hard and soft with soft, and let each solvent define its own acids and bases.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Period 3 Oxides and the Acid-Base Divide](https://lightmysky.com/learn/science/period-3-oxides-and-the-acid-base-divide-mt_3kC5hTavIQ)

## Opens up

- [Coordination Compounds: Naming, Geometry and Isomerism](https://lightmysky.com/learn/science/coordination-compounds-naming-geometry-and-isomerism-mt_rkHM3jL3Ri)