What exactly is basicity?

In my textbook, it’s given that the trend of the basicity of the elements of group 15 is $$\ce{NH3 > PH3 > AsH3 > SbH3 \ge BiH3}$$ Also, it’s said that the reducing character decreases down the group. (Which is the ability to lose $\ce{e-}$) It’s also give (on a different page) that $\ce{H3PO3}$ and $\ce{H3PO4}$ are di- and tri-basic respectively.

Phosphoric acid (presumably, a Brønsted-Lowry base) has a basicity of 3. This would imply that basicity is a property of acids. However, Ammonia (which is a Lewis base) has the highest basicity. Further, quickly Google search of the definition of basicity states: “Basicity is the number of hydrogen atoms replaceable by a base in a particular acid.”

My question is, how does Ammonia (a Lewis base) have a basicity (i.e. acid character)... shouldn’t it be a weak base? Or is my understanding of the definition of basicity wrong?

• Equivocation at its best. – DHMO Oct 12 '16 at 14:15
• – Mithoron Oct 12 '16 at 17:44

The second is somewhat confusing, and it is used to describe how many protons of an acid can be removed by neutralisation with base. For example, $\ce{HCl}$ has one acidic proton, so it is monobasic; $\ce{H2SO4}$ has two acidic protons, so it is dibasic.