In , the nitrogen is bound covalently to the three oxygens, and the complex has an overall of .

Sodium has a charge of , and is ionically bound to the  complex.

When put into water, the compound will dissociate into  and .

HCl is an ionic compound, while the other answer choices have only covalent bonds.

Covalent bonds form when two atoms share electrons, caused by an overlap of their orbitals. Each atom has a similar amount of "pull" on the electrons, preventing them from getting too close to one atom or the other and keeping them in the middle. This "pull" is the electronegativity. Atoms in covalent bonds have similar electronegativity values to keep the electrons in the center. When the electronegativities are slightly different, the electrons can lean toward one atom. The result is a polar bond, in which one atom is closer to the electrons (negative) and the other is farther (positive). Most non-metals have very high and very similar electronegativity values, and will readily form covalent bonds.

When the difference in electronegativity, or "pull," is too great, the electrons will be transferred from one atom to the other. The result is an ionic bond, which usually forms between a metal (low electronegativity) and a non-metal (high electronegativity).

There are two primary types of intramolecular bonds: ionic bonds and covalent bonds. In an ionic bond, an electron is transferred (donated) from one atom to the other, usually allowing both atoms to satisfy the octet rule. In a covalent bond, electrons are shared between two atoms in order to allow both to satisfy the octet rule. A covalent single bond consists of two shared electrons, while a double bond will consist of four and a triple bond will consist of six.

Covalent bonds usually form between two non-metals. Diatomic gases, CH₄, and NaOH are all examples of molecules that contain covalent bonds.

A polar covalent bond describes a bond between two atoms that share electrons unequally. This results from a difference in electronegativity between the atoms. This difference is enough to cause a dipole, but not enough to consitute an ionic bond.

A non-polar covalent bond is a bond between two atoms sharing ions equally due to similar electronegativities.

A double bond occurs when four electrons are shared between two atoms.

A coordinate covalent bond refers to a covalent bond in which one atom donates both shared electrons.

A nonpolar covalent bond occurs when two atoms share electrons equally. This happens when the electronegativities of each atom is relatively close to one another. For example, in water, oxygen is much more electronegative than hydrogen (3.5 and 2.1 respectively), oxygen keeps the electrons closer to its nucleus and results in uneven sharing of the electrons between itself and the two hydrogen atoms. This results in a net dipole in the molecule with the oxygen-end being slightly negative, and the hydrogen-end being slightly positive.