# Which of the following is a polyatomic ion: CH3COOH, CR2O7^2-, Na^+, or H2?

Dec 13, 2015

${\left[C {r}_{2} {O}_{7}\right]}^{2 -}$ is a polyatomic ion.

#### Explanation:

Polyatomic ions are ions with charge that are made up of two or more atoms.

For example:

• ${\left[{H}_{3} O\right]}^{+}$
• ${\left[S {O}_{4}\right]}^{2 -}$
• ${\left[C {O}_{3}\right]}^{2 -}$

Notice that all of these examples have a charge attached to them. This is what makes them ionic.

Also, all of these ions are made up of more than 2 ions.

Looking through the problem that was given, we have 4 different choices.

First, discard the ones that do not have a charge attached to them. ${H}^{2} \mathmr{and} C {H}_{3} C O O H$ do not have charges associated with them, so discard that.

Second, discard the ones that do not have two or more ions in them. $N {a}^{+}$ is a single sodium ion, so this can be discarded.

Finally, you are left with the last choice: ${\left[C {r}_{2} {O}_{7}\right]}^{2 -}$. This is your answer.