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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?

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The issue is that basicity has a couple of meanings and you have to be able to tell from the context what it means.

The first is logical, and is used to describe how basic a base is. That is the trend that you mentioned about the Group 15 hydrides. (In my experience, this is by far the more common usage of the word.)

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.

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  • $\begingroup$ So, basicity can mean both the strength of the acid as well as the base? Does the meaning change when talking about either a Lewis acid or Bromsted base? My bad.. the error is on my part and not my textbook. I'll edit it now. $\endgroup$ – Abhi2011 Oct 12 '16 at 13:54
  • $\begingroup$ It is not about the strength of the acid. It is simply a measure of how many protons can come off; it does not say anything about how difficult it is to get those protons off. Acetic acid and HCl are both monobasic, but acetic acid is a weak acid and HCl a strong acid. $\endgroup$ – orthocresol Oct 12 '16 at 13:56
  • $\begingroup$ Okay. My question came off wrong. Let me rephrase. $\endgroup$ – Abhi2011 Oct 12 '16 at 15:48
  • $\begingroup$ So, basicity can mean both how acidic an acid is as well as how basic a base is? Does the meaning change when talking about acids or bases in the Lewis acid-base theory or the Bromstead-Lowry acid base theory? $\endgroup$ – Abhi2011 Oct 12 '16 at 15:49
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    $\begingroup$ Basicity meaning number one applies to all kinds of bases, although it is mostly applied to Bronsted bases. The meaning does not change depending on the type of base. Basicity meaning number two only applies to Bronsted acids. $\endgroup$ – orthocresol Oct 12 '16 at 16:00

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