# Why are acids and bases are defined as both donors and acceptors of species in the acid-base theories? [closed]

So, a Brønsted-Lowry acid is a proton donor, and a Brønsted-Lowry base is a proton acceptor. On the other hand, a Lewis acid is an electron pair acceptor, and a Lewis base is an electron pair donor.

Why is it that in these acid-base theories, acids and bases are defined as both donors and acceptors of species? Or is there a part of acid-base theory I don't quite understand?

## closed as unclear what you're asking by Gaurang Tandon, airhuff, Tyberius, Todd Minehardt, MithoronMar 16 '18 at 16:17

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• I honestly don't understand your question. How else do you want acids or bases to be defined as? Some ions will always be donated into the solution, and some will always be accepted from it. What other way is there for acids or bases to function? – Gaurang Tandon Mar 16 '18 at 2:39

These are primarily old theories that were kind of evidence or observation based. These are not completely correct theories as there are may loop holes in them. For eg. many types of compounds with significant acidic and basic characters cannot be predicted by these theories. Also, to a certain extent the Bronsted and Lewis concepts are complementary.Wikipedia says - ""

A Lewis base is often a Brønsted–Lowry base as it can donate a pair of electrons to H+; the proton is a Lewis acid as it can accept a pair of electrons. The conjugate base of a Brønsted–Lowry acid is also a Lewis base as loss of H+ from the acid leaves those electrons which were used for the A—H bond as a lone pair on the conjugate base.

and

In 1923, Lewis wrote An acid substance is one which can employ an electron lone pair from another molecule in completing the stable group of one of its own atoms. The Brønsted–Lowry acid–base theory was published in the same year. The two theories are distinct but complementary.

Basically, proton is a $H^+$ ion and we all know from our simple understanding that compounds that give $H^+$ or proton are acids and those that accept $H^+$ or proton are bases.

• They're distinct alright, but complementary as mentioned. I prefer the Brønsted-Lowry theory though... It's easier to understand for me. – user60221 Mar 15 '18 at 20:40
• @KianStevens You know, its not absolutely fine I would see. For eg. if you are reading Friedel Crafts Alkylation you would come across the need of a Lewis acid like AlCl3 or BF3. If you are acquainted with the Lewis theory, you would face there as AlCl3 is not predicted by the Bronsted Lowry concept. – tatan Mar 16 '18 at 13:50