Question
Which property makes a molecule or ion a Lewis acid?
(a) Taking a proton from water.
(b) Taking an electron pair to make a chemical bond.
(c) Providing an electron pair to make a chemical bond.
(d) Giving a proton to water.
(e) Having multiple resonance electron-dot representations.
Answer
Choose (b). A Lewis acid accepts an electron pair to form a bond. The partner donating that pair is the Lewis base. Their bond-containing product is called an adduct.
Evaluate all five choices
| Part | What it describes | Why it is or is not the defining property |
|---|---|---|
| (a) | Accepting a proton from water | This is Brønsted–Lowry base behavior. |
| (b) | Accepting an electron pair for a bond | This is the Lewis acid definition. |
| (c) | Donating an electron pair for a bond | This defines a Lewis base. |
| (d) | Donating a proton to water | This describes a Brønsted–Lowry acid, a narrower category. It does not define every Lewis acid. |
| (e) | Having resonance representations | Resonance does not by itself determine acid/base behavior. |
A neutral molecule can qualify: accepts an electron pair at boron even though it has no proton to donate. An ion such as can also accept an electron pair. Positive charge is therefore not a required part of the definition.
In proton transfer, the base's electron pair bonds to the proton, so Brønsted–Lowry acid reactions fit within the Lewis framework. That overlap does not make (d) the general definition: (b) covers proton-free electron-pair acceptors too. Electron-pair donation into a shared bond should not be confused with a one-electron redox transfer.
Evidence boundary
The five choices are a normalized presentation of AP99-38 in the UT Austin-hosted Chemistry Questions — English Form. The page establishes that hosted version, not an independently verified original exam edition. Choice (b) is the general definition; explaining why (d) is narrower does not deny that proton-transfer acids also participate in Lewis acid–base chemistry.
Sources
These references support the concepts and methods used in the explanation above.