Which of the following is the ammonium ion?

• A. NH4+
• B. NH3
• C. NH4
• D. NH5+

Answer: (A)

When a base like ammonia gains a proton, it becomes a positively charged ion. Ammonia is NH3, with three N–H bonds and one lone pair. If the lone pair accepts a proton, you form four N–H bonds and remove the lone pair, giving NH4+ with a +1 charge. That extra proton is what makes it an ion.

So the ammonium ion is NH4+. The other possibilities describe ammonia (NH3) or species that aren’t the commonly observed ammonium ion (NH4 would be neutral, and NH5+ would require nitrogen to form five bonds, which doesn’t happen in usual chemistry).