Question #79c23

1 Answer
Ernest Z.
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, "NH"_4^+.

It has eight valence electrons.

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The neutral "N" atom originally had 5 valence electron, and each "H" atom originally had 1 valence electron, for a total of 9 valence electrons.

The positive charge arises because the 8 negative electrons in "NH"_4^+ cannot balance the positive charges of the 9 protons in the "N" and "H" nuclei.

Phosphate ion, "PO"_4^(3-), is a polyatomic anion.

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The phosphate ion has 32 valence electrons.

The neutral "P" and "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.

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