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Is $\ce{NH_3}$ an acid or a base? N has a higher electro-negativity (3.04) than hydrogen (2.20), therefore nitrogen attracts the bond's electrons easily. The reaction should produce $\ce{H^+}$ because the electrons of hydrogen are attracted by nitrogen.

According to my Chemistry book, my assumption seems to be wrong because $\ce{NH_3}$ is considered a base. Why am I wrong?

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    $\begingroup$ Oxygen has a higher electronegativity than nitrogen. Therefore, which reaction should be more likely: $$\ce{NH3 + H2O <=> NH4^+ + OH^-}$$ $$\ce{NH3 + H2O <=> H3O^+ + NH2^-}$$ This example only applies in water, of course, but $\ce{NH3}$ will act as a base in many other instances, also. $\endgroup$ – SendersReagent Mar 28 '16 at 11:22
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    $\begingroup$ Indeed, $\ce{NH3}$ is an acid and a base, like water and many other compounds. $\endgroup$ – Ivan Neretin Mar 28 '16 at 12:02
  • $\begingroup$ Ammonia is a base because it's lone pair on nitrogen accepta protons. Ammonia could act as a really weak acid. $\endgroup$ – Yunfei Ma Mar 28 '16 at 22:28
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The difference isn't that stark. However, you have to bear in mind the Configuration of electrons on the central nitrogen. We often will draw ammonia with the electron pair atop the nitrogen. That lone pair will readily accept a proton, making it a base.

To put it another way, ammonia is far more ready to accept a proton than to release one thanks to the lone pair.

Of course, it also depends on the environment it's in. But in general, you'll usually see ammonia (or just nitrogens in compounds) referred to as basic

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