Three models of acids and bases
Compare the three acid-base models — Arrhenius, Brønsted–Lowry, and Lewis — and learn which to use for aqueous solutions, proton transfers, and electron-pair reactions.
Quick answer
Arrhenius: acids make H⁺ and bases make OH⁻ in water. Brønsted–Lowry: acids donate protons (H⁺), bases accept them. Lewis: acids accept electron pairs, bases donate them — the broadest definition.
Example: HCl is an acid in all three models; NH₃ is a base in all three (it accepts H⁺ from water, and donates its lone pair); BF₃ is an acid only in the Lewis model — it accepts an electron pair but has no proton chemistry.
What you will learn
- State the definitions of acids and bases under all three models.
- Classify a substance under each model with examples.
- Explain why Brønsted–Lowry replaced Arrhenius for most aqueous chemistry.
- Give an example that only the Lewis model explains.
Arrhenius: acids and bases in water
The oldest model, from Svante Arrhenius (1884), is refreshingly concrete:
- Acid: produces H⁺ in water (HCl → H⁺ + Cl⁻)
- Base: produces OH⁻ in water (NaOH → Na⁺ + OH⁻)
It explains strong acids, strong bases, and neutralization neatly. But it fails quickly: NH₃ makes basic solutions without containing OH⁻, and it says nothing about reactions outside water. Arrhenius is a good first model — and a stepping stone, not the destination.
Brønsted–Lowry: the proton-transfer view
Brønsted and Lowry (1923) shifted the focus from the solvent to the proton (H⁺):
- Acid: proton donor
- Base: proton acceptor
Now NH₃ is a base because it accepts a proton from water: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻. The model naturally produces conjugate pairs — an acid and the base formed when it loses its proton (HCl/Cl⁻, NH₄⁺/NH₃). It also handles non-aqueous solvents and is the workhorse model of general chemistry: buffers, titrations, and Ka all speak Brønsted–Lowry.
Lewis: the electron-pair view
G. N. Lewis (1923) went one level deeper, to electrons:
- Acid: electron-pair acceptor
- Base: electron-pair donor
This covers everything the other models do, plus reactions with no protons whatsoever: BF₃ + :NH₃ → F₃B–NH₃, where ammonia's lone pair flows into boron's empty orbital. Metal cations (Al³⁺, Fe³⁺) are Lewis acids — which is exactly why their aqueous solutions are acidic. Organic reaction mechanisms are written almost entirely in Lewis language: arrows showing electron pairs moving.
Choosing the right model
Think of the models as nested boxes: Arrhenius ⊂ Brønsted–Lowry ⊂ Lewis. For aqueous proton chemistry, Brønsted–Lowry is the default. For gas-phase, non-aqueous, or metal-centered chemistry, reach for Lewis. Arrhenius remains useful shorthand for 'strong acid in water' contexts.
A substance's label can depend on the model — and on the reaction. Water is an Arrhenius neutral, a Brønsted acid toward NH₃ and a Brønsted base toward HCl, and a Lewis base toward metal ions. 'Acid' and 'base' are roles, not permanent identities.
Common mistakes
They nest: every Arrhenius acid/base is a Brønsted–Lowry acid/base, and every Brønsted–Lowry acid/base is a Lewis acid/base. Each model widens the definition; none overturns the previous one.
Only under Arrhenius and Brønsted–Lowry. A Lewis acid like BF₃ or Al³⁺ contains no hydrogen at all — it is an acid because it accepts an electron pair. 'Must contain H' is a model-limited rule.
You already use it: every metal cation in water (Al³⁺, Fe³⁺) acts as a Lewis acid, which is why those solutions are acidic. Coordination chemistry and much of organic chemistry run on Lewis ideas.
Real-world applications
Acid-base theory: choosing the right model for aqueous versus non-aqueous chemistry.
Organic chemistry: Lewis theory explains reactions with no protons at all.
Biochemistry: proton transfers in enzyme active sites are Brønsted–Lowry reactions.
Industrial catalysis: Lewis acids like AlCl₃ drive Friedel–Crafts reactions.
Frequently asked questions
Which model should I use on an exam?
Use Arrhenius for simple aqueous strong acid/base questions; Brønsted–Lowry whenever proton transfer matters (buffers, conjugate pairs, weak acids); Lewis for non-aqueous systems, metal complexes, and organic mechanisms. When in doubt, Brønsted–Lowry is the default in general chemistry.
Is ammonia a base under all three models?
Yes — NH₃ has no OH⁻, yet its solution is basic. Brønsted–Lowry explains it: NH₃ accepts a proton from water (NH₃ + H₂O ⇌ NH₄⁺ + OH⁻). Arrhenius cannot explain this without awkward workarounds, which is why Brønsted–Lowry replaced it for most purposes.
Does the broadest model make the others obsolete?
No. The Lewis model covers reactions with no protons at all — BF₃ + NH₃ → F₃B–NH₃ has no H⁺ transfer, so Brønsted–Lowry is silent, but Lewis sees the electron-pair donation clearly. Each model extends the previous one's reach.
Is every Brønsted acid also a Lewis acid?
Every Brønsted–Lowry acid is a Lewis acid (donating a proton means accepting the electron pair it was bonded to), but not every Lewis acid is a Brønsted–Lowry acid (BF₃ has no proton to donate). The categories nest like Russian dolls.