# Question #79c23

Feb 11, 2014

Polyatomic ions have a charge because they have more or fewer valence electrons than the neutral atoms that make up the ions.

#### Explanation:

The most common polyatomic cation is the ammonium ion, ${\text{NH}}_{4}^{+}$.

It has eight valence electrons.

The neutral $\text{N}$ atom originally had 5 valence electron, and each $\text{H}$ atom originally had 1 valence electron, for a total of 9 valence electrons.

The positive charge arises because the 8 negative electrons in ${\text{NH}}_{4}^{+}$ cannot balance the positive charges of the 9 protons in the $\text{N}$ and $\text{H}$ nuclei.

Phosphate ion, ${\text{PO}}_{4}^{3 -}$, is a polyatomic anion.

The phosphate ion has 32 valence electrons.

The neutral $\text{P}$ and $\text{O}$ atoms originally had only 5 + 4×6 = 29 valence electrons.

The negative charge is caused by the three extra electrons that came from some outside source, e.g., metal atoms that donated their valence electrons and formed cations.