How do polyatomic ions compare with covalently bonded molecules?

There are both similarities and differences between molecules and polyatomic ions. First for the similarities:

Atoms that are covalently bonded within a molecule are held together by pairs of electrons that are shared between two atoms. The atoms that make up a polyatomic ion are also covalently bonded, or held together by shared pairs of electrons. In both molecules and polyatomic ions the atoms are separated from each other when a chemical reaction takes place....

There are both similarities and differences between molecules and polyatomic ions. First for the similarities:

Atoms that are covalently bonded within a molecule are held together by pairs of electrons that are shared between two atoms. The atoms that make up a polyatomic ion are also covalently bonded, or held together by shared pairs of electrons. In both molecules and polyatomic ions the atoms are separated from each other when a chemical reaction takes place. Phase changes such as melting and vaporizing don't involve breaking of covalent bonds. 

There are two main differences between molecules and polyatomic ions:

1. Molecules are electrically neutral while polyatomic ions have a net charge. In the process of the ion forming one or more electrons were either gained or lost. Most polyatomic ions have an excess of electrons and are therefore negative, such as NO3(-) and SO4(2-). There is one common positive polyatomic ion, NH4(+), which has one less electron than it does protons. 

2. Molecules exist as molecular compounds, which generally have low melting and boiling points because attractions between individual molecules are weak. Polyatomic ions exist in ionic compounds. A polyatomic ion is attracted to one or more oppositely charged ions, forming a compound that exists as a crystal lattice of repeating positive and negative ions.  Ionic compounds have high melting points because a lot of energy is needed to overcome the strong attraction between positive and negative ions. An example of a polyatomic ionic compound is sodium nitrate, NaNO3, which is made of the positive ion Na+ and the negative polyatomic ion NO3(-).