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]
Phosphodiester bond Phosphodiester bonds are central to all life on Earth,[fn 1] as they make up the backbone of the strands of nucleic acid. In DNA and RNA, the phosphodiester bond is the linkage between the 3' carbon atom of one sugar molecule and the 5' carbon atom of another, deoxyribose in DNA and ribose in RNA. Strong covalent bonds form between the phosphate group and two 5-carbon ring carbohydrates (pentoses) over two ester bonds.