Which of the following is a
polyatomic ion? (A) F−, (B) O2, (C) NO3−, (D) SO2,
or (E) Mg2+.
The first thing to note is that
we’re being asked to find a polyatomic ion. The poly- in polyatomic means more
than one. So, we’re looking for an ion that
contains more than one atom. So, any answer with only one atom,
monatomic, can be ruled out. This rules out F− and Mg2+. Next, we’re looking for an ion. An ion is something that has a
nonzero charge. We can spot an ion because it will
have either a positive or negative charge notation.
For example, in answer (A), F−
shows a negative charge on our fluorine atom. This makes it an ion. So, let’s look at our three
remaining answers and see if any of them have a charge. Both O2 and SO2 are neutral, so we
can rule these out. Answer (C) NO3− does indeed have a
charge. A negative charge means that we can
call this an anion. Remember that a positive charge is
called a cation. So, here, we have our answer. The polyatomic ion in this question
is answer (C) NO3−, the nitrate anion. Out of interest, you can find the
nitrate anion in nitric acid, HNO3.