Donating electrons versus protons [ENDORSED]

[Mary Becerra 2D]

How can you tell when an atom is donating a proton versus an electron? Does it work as in when a proton is donating electrons it is always donating protons? Therefore a Lewis base is always a Bronsted acid?

[Nora 1F]

A Lewis acid accept electrons and Lewis bases donate electrons. Bronsted acids donate protons while Bronsted bases accept protons. A Lewis base does not necessarily have to be a Bronsted acid. When only electrons are being transferred (no protons), you are dealing with Lewis definitions.

[Jonathan Tangonan 1E]

I think what is important to clarify in this question is that there are two several definitions in the textbook which define both acids and bases. Lewis acids and bases involve the giving and accepting of electron pairs while Bronsted Acids and Bases involve the donation and accepting of a proton (H+). Typically, if you were to draw out the lewis structures of the reaction you can identify which molecules or substances are donating or accepting an electron pair or a proton.

[Yashaswi Dis 1K]

I think an easier way to think of it is like this: If a molecule has electron lone pairs on it, according to the Lewis definition, it will be a Lewis base because it is donating an electron pair, while the Lewis Acid is the molecule that accepts the electron lone pair. So for instance, if I have:

NH3 (aq.) + H20 (l) <----> (NH4^+1) (aq.) + (OH^-1) (aq.)

The Lewis base is NH3 because it has a electron lone pair on the N that the H from the H20 can accept. Thus, H20 is a Lewis Acid.
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Bronsted-Lowry definitions (another way to think of the reaction in terms of H^+1 as the proton...kind of like thinking in terms of protons instead of electrons now...a new shift in perspective you may call it):

Bronsted Acid: An acid that is a proton donor. In this case, it's H20 because it donates a H to Ammonia for the underlying chemistry mentioned above.

Bronsted Base: A base is a proton acceptor. In this case, it's NH3 because it accepts the H atom from water, forming Ammonium.

Hopefully, given that example and the definitions, it is clearer!

Good luck on the final!

Sincerely,
Yashwi

[Sohini Halder 1G]

I also think it is important to differentiate between Lewis, Bronsted, and Arrhenius definitions of acids and bases.

Arrhenius definition states that a compound that dissociates in water to form hydronium ions is an acid, so a compound like HCL is an Arrhenius acid as well as a Bronsted acid and a Lewis acid.

Arrhenius bases are compounds that dissociate in water to form OH- ions. So, even though NH3 is a Lewis base because it is not an Arrhenius base.