which molecule or ion needs only two resonance structures to describe its bonding
Resonance (chemistry) The ozone molecule is represented by two resonance structures. In reality the two terminal oxygen atoms are equivalent and the hybrid structure is drawn on the right with a charge of − 1⁄2 on both oxygen atoms and partial double bonds with a full and dashed line and bond order  1 1⁄2.[15][16]
Degrees of freedom (physics and chemistry) In three-dimensional space, three degrees of freedom are associated with the movement of a particle. A diatomic gas molecule has 5 degrees of freedom[dubious – discuss]. This set may be decomposed in terms of translations, rotations, and vibrations of the molecule. The center of mass motion of the entire molecule accounts for 3 degrees of freedom. In addition, the molecule has two rotational degrees of motion and one[dubious – discuss] vibrational mode. The rotations occur around the two axes perpendicular to the line between the two atoms. The rotation around the atom–atom bond is not a physical rotation[dubious – discuss]. This yields, for a diatomic molecule, a decomposition of: