IMAT 2011 Q66 [Electron Number]

What is the total number of electrons in an ammonium ion, {NH_4^+}?

A. 8
B. 9
C. 10
D. 11
E. 12

There may be more than one way to solve this question but the fastest one is to recall the first two rows of the periodic table from memory.

We can remember that each Hydrogen has a single electron and that Nitrogen is the first element of group 15 which means that it has 7 electrons. In total: 4 hydrogen electrons and 7
nitrogen electrons equal a total of 11 electrons in the ammonium, -1 electron since in this case, we have an anion: 10 electrons in total. Hence, the answer is C.