# Why isn't ammonium strong acid, since it's charged and ammonia is neutral? [duplicate]

Why is ammonium such a weak acid $(\mathrm pK_\mathrm a = 9.24)$, if $\ce{NH4+}$ has a positive charge on nitrogen, while $\ce{NH3}$ is neutral. It means that conjugate base (ammonia) is really stable, which makes conjugate acid a strong acid.

Another argument why ammonium should be strong acid is comparison with hydronium ion $\ce{H3O+}$. Both nitrogen (EN = 3.04) and oxygen (EN = 3.44) are significantly electronegative atoms and "don't like" to be positively charged. That explains pretty good hydronium ion $\mathrm pK_\mathrm a = -1.74$, but why is then ammonium $\mathrm pK_\mathrm a$ is just $9.24$?