Main Characterstics or Lewis/Bronsted Definition
Key Questions
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Both definitions are correct. Neither definition is wrong.
The definitions are related, but they refer to different phenomena.
A Brønsted acid is a proton donor. A Brønsted base is a proton acceptor.
A Lewis acid is an electron pair acceptor. A Lewis base is an electron pair donor.
If you want to discuss a reaction in terms of proton transfers, you use the Brønsted definitions.
If you want to discuss a reaction in terms of electron pair transfers, you use the Lewis definitions.
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A Lewis acid is a molecule that accepts an electron pair, whereas a Lewis base is a molecule that donates an electron pair.
The implications of this definition are much wider than more traditional definitions.
For instance,
ammonia is a Lewis acid and boron trifluoride is a Lewis base in this example.