Polyatomic ions:

Polyatomic ions are electrically charges collections of two or more atoms.

Typically in beginning classes people are not expected to predict polyatomic ion formulas. Polyatomic ions usually have a charge because the collection of atoms has either gained an extra electron or else it has lost an electron.

Most of the common inorganic polyatomic ions have negative charges. This means the ion has more electrons than the neutral atoms that are in the ion. A polyatomic ion like phosphate, PO₄^3- has three extra electrons beyond the electrons on the P and O atoms. The extra electrons are shared by all the atoms in the polyatomic ion. These electrons are sort of smeared around the atoms in the ion.

If you count up the valence electrons in the phosphate ion you will see there are 32 electrons. The neutral P and O atoms originally had only 29 valence electrons. The negative charge is due to the extra three electrons that came from some outside source like metal atoms that formed cations. The charge is written outside the brackets to indicate the charge is spread on the ion as a whole.

The single most common positive polyatomic ion is the ammonium ion, NH₄¹⁺, (note the -ium ending on the positive ion). Positive polyatomic ions are short electrons.Polyatomic ions usually have what is called a "central" atom. This is what it says.

The central atom is physically in the middle of the ion. The other atoms are attached to the central atom. Normally the oxygen atoms are attached to the central atom and radiate out like spokes.

Hydrogens are attached to the oxygens. The hydrogens are "terminal" atoms far from the central atom. The oxygens act as a bridge between the central atoms and the hydrogens. This will be seen later when you study acids like HNO₃ or H₂SO₄. Here are ball and stick models for phosphoric acid, H₃PO₄, and phosphorus acid, H₃PO₃. Notice the exception to the generalization that says hydrogen atoms are attached to the oxygens.