# Question #79c2b

##### 1 Answer
Mar 19, 2014

The definition of an ion is an atom or molecule that has a net charge. This is the result of having different numbers of electrons and protons.

Example:
${H}_{3} {O}^{+}$ is a polyatomic ion because it has a total of 11 protons (8 from $O$ and 1 from each $H$) but only 10 electrons.

Another example is the hydroxide ion ($O {H}^{-}$)

Oxygen has 6 valence electrons and hydrogen has one valence electron. It's not possible to create a Lewis structure for this ion and meet the octet rule for oxygen without adding one more electron. Adding one electron allows for a Lewis structure to meet the octet rule for oxygen and gives the ion a -1 charge. The ion has a single bond between the O-H and three non-bonding electron pairs around the oxygen.