orbitals into which they can accept an electron pair, or are positively polarized atoms.
BRIDGE
Acids and bases are critically important material in organic chemistry, biochemistry, and
general chemistry. The most extensive coverage of acids and bases is in Chapter 10 of MCAT
General Chemistry Review.A Lewis base is defined as an electron donor in the formation of a covalent bond. Lewis bases also
tend to be nucleophiles, which we will touch on in the next section. Lewis bases have a lone pair of
electrons that can be donated, and are often anions, carrying a negative charge.
When Lewis acids and bases interact, they form coordinate covalent bonds—covalent bonds in
which both electrons in the bond came from the same starting atom (the Lewis base), as shown in
Figure 4.1.
Figure 4.1. Lewis Acid–Base ReactionsIn the Brønsted–Lowry definition, an acid is a species that can donate a proton (H+); a base is a
species that can accept a proton. Some molecules, like water, have the ability to act as either
Brønsted–Lowry acids or bases, making them amphoteric. Water can act as an acid by donating its
proton to a base, and thus becoming its conjugate base, OH–. However, water can also act as a base
by accepting a proton from an acid to become its conjugate acid, H 3 O+. The degree to which a
molecule acts as an acid or a base is dependent upon the properties of the solution—water can only
act as a base in an acidic solution, and only as an acid in a basic solution. Other examples of
amphoteric molecules include Al(OH) 3 , , and .