--- title: "Acid-Base Models Beyond Bronsted: Lewis, Solvents and Hard and Soft" description: "Lewis acids accept electron pairs and Lewis bases donate them, including in reactions without proton transfer. Hard and soft classifications help predict relative binding preferences. The solvent-syst" canonical: https://lightmysky.com/learn/science/acid-base-models-beyond-bronsted-lewis-solvents-and-hard-and-soft-mt_jarui-9yfx source: https://lightmysky.com/learn/science/acid-base-models-beyond-bronsted-lewis-solvents-and-hard-and-soft-mt_jarui-9yfx.md retrieved: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Acid-Base Models Beyond Bronsted: Lewis, Solvents and Hard and Soft Lewis acids accept electron pairs and Lewis bases donate them, including in reactions without proton transfer. Hard and soft classifications help predict relative binding preferences. The solvent-system model defines acids and bases using a solvent's characteristic ions. 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 specified Lewis acids and donor atoms as hard or soft and predicts relative binding preferences under comparable conditions - Uses hard and soft preferences as one factor in explaining carbonate and sulfide mineral associations - Applies the solvent-system definition to a named non-aqueous solvent ## Lesson: Acids without protons A Lewis base supplies an electron pair for a bond; a Lewis acid accepts it. In NH3 + BF3, nitrogen supplies its lone pair to boron: NH3 is the base and BF3 is the acid. No proton transfers. Brønsted-Lowry acids donate protons and bases accept them. That definition also applies outside water. **Example.** Metal ions can accept ligand pairs. In Ag+ + 2NH3 -> [Ag(NH3)2]+, Ag+ accepts pairs from two NH3 bases. The neutral ligands add no charge: +1 + 2(0) = +1. Product charge does not identify every reactant's Lewis role. Follow the donated pairs. Hard species resist polarisation; soft species are more easily polarised. Hard ions often have high charge density, but neutral bases can also be hard. Mg2+, Al3+ and H+ are hard acids; Ag+, Hg2+ and Pt2+ are soft acids. F-, O^2- and NH3 are hard bases; I- and S^2- are soft bases. For comparable partners, HSAB predicts hard-hard and soft-soft preferences, not a universal ranking of binding strengths. *(drawing: These are classifications, not measured binding strengths. A base's donor atom matters.)* Magnesite, MgCO3, pairs hard Mg2+ with carbonate's hard oxygen donors. Cinnabar, HgS, pairs soft Hg2+ with soft sulfide. These associations fit HSAB. The rule cannot predict mineral abundance or an ore deposit by itself: solubility, available elements and chemical conditions also matter. Magnesium occurs in other minerals too. The solvent-system model starts with self-ionisation. Liquid ammonia has 2NH3 ⇌ NH4+ + NH2-. An acid increases its characteristic cation, NH4+; a base increases its characteristic anion, NH2-. Neutralisation gives NH4+ + NH2- -> 2NH3. Water instead uses H3O+ and OH-. Brønsted-Lowry and Lewis models can still describe reactions in either solvent. **Recap.** Lewis roles follow electron pairs. HSAB predicts relative preferences. Solvent-system roles follow the solvent's characteristic ions. *Lesson, questions and flashcards written for this dataset.* ## 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)