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Why does NH3 act as Lewis base?
According to the Lewis definition of , a compound isn't acidic or basic until it does something. In other words, you can't just say that "X compound is a Lewis acid" unless you've seen it act as an acid in some chemical reaction - in which case you'd say that "X compound is a Lewis acid in this particular reaction."
For ## NH_3## to be a Lewis base, it would have to react in such a way that a lone pair on it attached itself to another atom or molecule. It turns out that ammonia tends to very readily donate its electron pair to other (most notably when it pulls hydrogen ions from other compounds), which is why we usually think of it as a Lewis base.