A Brønsted acid donates a proton, while a Lewis acid accepts an electron pair. The definitions describe different ways of identifying acids: every Brønsted acid–base reaction involves proton transfer, whereas a Lewis acid–base reaction involves an electron-pair interaction and need not involve a proton at all.
Why Chemistry Has Two Acid Definitions
The familiar idea of an acid as a substance that gives up hydrogen ions is useful, but it does not cover every reaction chemists classify as acid–base chemistry. The Brønsted–Lowry definition focuses on proton transfer and applies broadly to reactions in water and other solvents. The Lewis definition, introduced by chemist Gilbert N. Lewis, focuses instead on how electron pairs are donated and accepted when a bond forms.
These definitions are not competing explanations of the same limited set of reactions. They work at different levels of breadth. A proton donor is also a Lewis acid in the reaction where it accepts an electron pair from a base as the proton attaches to it. But some Lewis acids have no proton to donate, so they are not Brønsted acids.
What Is a Brønsted Acid?
A Brønsted acid is a proton donor. A proton is the positively charged hydrogen nucleus, written H+ in many chemical equations. A Brønsted base is a proton acceptor. The acid and base are identified by what happens during a particular reaction, rather than simply by a label attached to a substance in every possible setting.
For example, when hydrogen chloride reacts with water, it transfers a proton to a water molecule:
HCl + H2O → H3O+ + Cl−
Hydrogen chloride is the Brønsted acid because it donates the proton. Water is the Brønsted base because it accepts that proton. The products are hydronium and chloride. In water, writing a free H+ as the product is often a shorthand; the proton is associated with water molecules, with hydronium serving as a useful representation.
After donating a proton, an acid becomes its conjugate base. After accepting one, a base becomes its conjugate acid. Thus, in this example, chloride is the conjugate base of hydrogen chloride, and hydronium is the conjugate acid of water. These paired forms help students track proton movement in reversible reactions.
What Is a Lewis Acid?
A Lewis acid accepts an electron pair to form a bond; a Lewis base donates an electron pair. The electron pair often comes from a lone pair, meaning a pair of valence electrons not already used in a bond. The resulting bond is commonly described as a coordinate covalent bond: both bonding electrons were supplied by the base when the bond formed.
A standard example is the reaction between boron trifluoride and ammonia:
BF3 + NH3 → F3B←NH3
Ammonia donates a lone pair on nitrogen, and boron trifluoride accepts it. Boron in BF3 has an incomplete valence shell, so it can accept electron density from a suitable donor. Ammonia is the Lewis base; BF3 is the Lewis acid. No proton moves in this reaction, so it is a clear example of Lewis acid–base chemistry that is not Brønsted acid–base chemistry.
The Key Difference at a Glance
| Feature | Brønsted–Lowry | Lewis |
|---|---|---|
| Acid definition | Donates a proton | Accepts an electron pair |
| Base definition | Accepts a proton | Donates an electron pair |
| What to track | Movement of H+ | Donation and acceptance of an electron pair |
| Must a proton be present? | Yes, a proton is transferred | No |
| Illustrative reaction | HCl transfers a proton to water | NH3 donates a pair to BF3 |
| Relative scope | A specific kind of acid–base reaction | A broader electron-pair framework |
The key test is therefore not whether a substance contains hydrogen. Ask what the reaction shows: proton transfer points to the Brønsted definition; electron-pair acceptance points to the Lewis definition. If both occur, the same reaction can be described using both frameworks.
How the Definitions Fit Together
Consider the reaction of a proton with ammonia:
H+ + NH3 → NH4+
Under the Brønsted–Lowry definition, H+ is the acid because it is donated to ammonia, which accepts it as the base. Under the Lewis definition, the proton accepts a lone pair from nitrogen, so it is the Lewis acid, and ammonia is the Lewis base. One reaction therefore satisfies both descriptions.
This overlap explains why introductory courses often begin with proton transfer and then introduce Lewis theory to cover additional cases. The Lewis framework does not make the Brønsted framework wrong or obsolete. It generalizes the electron-pair pattern behind many acid–base interactions, while Brønsted terminology remains especially direct when the central event is a proton moving between species.
Concrete Examples and How to Classify Them
Use the reaction, not just a memorized list of substances, to identify the roles. A useful method is to write down what changes and then name the acid and base according to the definition being used.
- Hydrogen chloride and water: HCl gives a proton to H2O. HCl is the Brønsted acid and water the Brønsted base. The water molecule supplies an electron pair to bond to the proton, so the interaction also fits Lewis language.
- Boron trifluoride and ammonia: NH3 supplies a lone pair to BF3. BF3 is the Lewis acid and NH3 the Lewis base. Since no proton is transferred, these are not Brønsted roles in this reaction.
- Metal ions and water molecules: A metal ion can accept an electron pair from water molecules that coordinate to it. This is a Lewis acid–base interaction. Some metal ions also make coordinated water more likely to lose a proton, but that is a related process and should not be confused with the initial electron-pair donation.
The last example illustrates why chemical contexts can involve more than one step. A species may participate in Lewis coordination and also affect proton-transfer behavior. Describe the actual event under discussion instead of assigning one universal label without reference to the reaction.
Common Mistakes Students Make
A frequent mistake is to assume that “acid” always means a proton donor. That is true by definition for a Brønsted acid, but Lewis acids can accept electron pairs without donating any proton. BF3 is the classic reminder that an acid need not contain hydrogen.
Another mistake is to treat the two definitions as mutually exclusive. A proton can accept an electron pair as it forms a bond, so a proton-transfer reaction can also be interpreted as a Lewis interaction. The labels answer different questions about the same chemistry.
Students may also call a substance a base merely because it has a lone pair. A lone pair can make electron-pair donation possible, but whether a species acts as a Lewis base depends on the reaction and its partner. Similarly, the presence of hydrogen alone does not establish that a compound is acting as a Brønsted acid; it must donate a proton in the reaction being analyzed.
Finally, do not confuse a Lewis acid with an oxidizing agent. A Lewis acid accepts an electron pair to form an interaction or bond. Oxidation–reduction chemistry concerns electron transfer and changes in oxidation states. The ideas can intersect in complex chemistry, but the terms are not interchangeable.
How to Remember the Difference
Connect “Brønsted” with a proton being passed, and “Lewis” with a pair of electrons being accepted. For a Brønsted question, mark the hydrogen that changes partners. For a Lewis question, follow the curved arrow from the electron-pair donor toward the electron-pair acceptor. Curved arrows show electron movement, not the movement of a proton by itself.
A quick classification routine is:
- Check whether a proton changes from one species to another.
- If it does, identify the donor as the Brønsted acid and the acceptor as the Brønsted base.
- Then check whether a species donates an electron pair to another species.
- If so, identify the pair donor as the Lewis base and the pair acceptor as the Lewis acid.
This approach is more reliable than trying to memorize every substance as permanently “an acid” or “a base.” Many substances can play different roles in different reactions.
Frequently Asked Questions
Is every Brønsted acid also a Lewis acid?
In a proton-transfer reaction, the proton acceptor donates an electron pair to the proton, so the proton-donating species can also be described as a Lewis acid in that interaction. The two definitions emphasize different features of the event.
Can a Lewis acid be a Brønsted acid?
Yes, if it also donates a proton in the reaction being considered. But Lewis acidity alone does not make a substance a Brønsted acid. A species such as BF3 accepts an electron pair without transferring a proton in its reaction with ammonia.
Is water a Brønsted acid or base?
It can be either. Water accepts a proton from HCl in one reaction and can donate a proton to a suitable base in another. Its role depends on which reaction and direction are being considered.
Why is the Lewis definition considered broader?
It includes electron-pair acceptance that occurs without proton transfer, such as BF3 accepting a lone pair from ammonia. Brønsted acid–base chemistry, by definition, requires a proton to be transferred.
What is the shortest way to distinguish them on an exam?
Look for the event specified in the question. Proton donation means Brønsted acid; electron-pair acceptance means Lewis acid. A reaction may qualify under both definitions, so state which feature you are using.
Why Both Definitions Matter
Brønsted–Lowry language is often the clearest choice for reactions involving proton transfer, conjugate acid–base pairs, and acid strength in a stated solvent. Lewis language is useful when explaining bond formation involving electron-pair donors and acceptors, including reactions with electron-deficient molecules or metal ions. Together, the definitions give students a more flexible way to describe chemical reactivity: one tracks protons, and the other tracks electron pairs.