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There are 2 approaches scientist use to describe acids and bases. These approaches take the names of the people who developed the theories of each approach.

Brønsted (-Lowry) Acids and bases

The Brønsted (-Lowry) approach to describing acids and bases revolves around the transfer of a H⁺ ion (or equally a proton). In this approach;

• an acid is described as being a H⁺ ion or proton donor.
• a base is described as being a H⁺ ion or proton acceptor.

To take a simple example of HCl reacting with NaOH. The HCl can be described as a proton donor and the NaOH can be described as a proton acceptor. 

Lewis Acids and bases

The Lewis approach to describing acids and bases revolves around pairs of electrons to form covalent bonds. In this approach;

• an acid is described as being an electron pair acceptor.
• a base is described as being an electron pair donor.

The Lewis approach to describing acids and bases allows a broader approach to describing acids and bases and allows an explanation of why particular reactions occur. For example, the reaction of BF₃ with NH₃. The Brønsted (-Lowry) approach may lead to the incorrect description of which way around to label the two reagents. 

Here the BF₃ acts as an electron pair acceptor (Lewis acid) and NH₃ acts as an electron pair donor (Lewis base).  It is easier to get to this conclusion if you analyse the valence electrons around each central atom. By doing this you can see BF₃ has room to accept the 2 valence electrons form NH₃.

Note; valence electrons are electrons in an atoms outer shell. 

The shape and nature of the orbitals with in molecules can also be a useful tool to fully understand what makes a Lewis acid/base. To learn more about orbitals see the chemBAM page.

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