Why is the acid always the electron withdrawing group and the base always the electron donating group?

Question

Why is the acid always the electron withdrawing group and the base always the electron donating group?

Answer 1

If you are considering the Bronsted-Lowry theory, then an acid is defined as a "proton donor".  A proton is a H⁺, which would draw electrons to it.  A base is defined as a proton acceptor.  Using similar reasoning, a base would either be a negatively charged ion (anion) or a species with one or more unshared electron pairs.  Having 'extra' electrons would (sort of) make it a 'donor' of electrons.

However, we rarely look at electron donation and/or accepting in the B/L theory.  But the Lewis theory does - just opposite of the way you're stating it in the question.  In the Lewis theory an acid is defied as an electron pair acceptor (thus, H⁺ or any species that has less than an octet of electrons, such as BCl₃), and a base is defined as an electron pair donor (thus, any anion or any species with unshared electron pairs, such as NH₃)