What is a polyatomic ion?

A polyatomic ion is simply a collection of covalently bound atoms, with a formal excess or deficiency of electrons......... Nitric acid is a NEUTRAL species, and we could draw the Lewis Structure as.....

#H-O-stackrel(+)N(=O)-O^(-)#.

Around the leftmost oxygen atom, there are 6 valence electrons, and 2 inner core electrons..... And thus with 2 inner shell electrons (i.e. the #1s^2# set), there are 8 electrons to match the nuclear charge, #Z=+8#, and this oxygen is formally neutral.

For sulfate, we have a similar distribution......

#(O=)_2S(-O^-)_2#.............

And for ammonium, we have a quaternized nitrogen, #NH_4^+#, and a formally positively charged ion...........

Just to add that for the purposes of assigning formal charge, we can go back to very old ideas that we learn when are introduced to bonding. In a covalent bond, electrons ARE SHARED between nuclei. An ionic bond is between a formal anion, and a formal cation, and thus involves the prior TRANSFER of electron.

If we take say methane, with FOUR #C-H# covalent bonds, we split this up to give #4xxdotH#, and a carbon atom with 4 valence electrons. The hydrogen ATOMS are neutral, because they have a positive nuclear charge, and also an electronic charge. And hence the hydrogens are neutral.

Likewise, the carbon atom claims 4 electrons from the four #C-H# bonds, and has 2 inner core electrons (i.e. formally the #1s^2# electrons. And so each carbon has 6 electrons, and there are 6 positive nuclear charges. The carbon atom in methane is also thus electronically neutral.

A polyatomic ion is a charged chemical species with more than one type of atom present.

Polyatomic ions are used worldwide on a daily basis, some common ones being nitrate (#"NO"_3^-#), perchlorate (#"ClO"_4^-#), and ammonium (#"NH"_4^+#).

Polyatomic ion species often bond chemically with metals to create simple ionic compounds, consisting of a geometric array of ions.

You'll be using polyatomic ions throughout your chemistry career, so it wouldn't hurt to know a bunch of them by memory; here is a list of some polyatomic ions:

Don't stress yourself trying to memorize them, a lot comes with experience and using them over and over!

An important thing to know about polyatomic ions is that they are (most of the time) considered as individual species during chemical reactions. That is to say, most of the time they will remain in that form during a chemical reaction, and won't transform into another substance.

Here's an example:

Suppose you're mixing a solution of #"NaCl# (sodium chloride) with #"Pb(CH"_3"COO)"_2# (lead acetate).

The chemical reaction is

#"NaNO"_3 (aq) + "Pb(CH"_3"COO)"_2 (aq) rarr "CH"_3"COONa" (aq) + "PbCl"_2 (s)#

This is a precipitation reaction, one in which a very low-soluble ("insoluble") salt is formed in aqueous solution.

As you can see, the polyatomic ion #"CH"_3"COO"^-# (acetate ion, also written as #"C"_2"H"_3"O"_2^-#) did not change, but actually remained in solution as dissolved ions (which you'll learn about when you discuss reactions in aqueous solution).

The preciptation of #"PbCl"_2# was the drive of the reaction, and if no precipitate forms, no reaction occurs (like I said, you'll learn this when you talk about aqueous solution reactions).

Keep in mind this was just an overview of the basics of what polyatomic ions are, and I don't expect this to be your sole source of information, but maybe some extra information you originally din't get.