D18 Resonance Structures and Sigma/Pi Bonds

what are sigma bonds?

this is the strongest type of covalent bond that is created by the head-on overlap of atomic orbitals between the two atoms involved in the bond. any single covalent bond between atoms is a sigma bond. in a double bond, one of the two bonds is a sigma bond. in a triple bond, one of the three bonds is a sigma bond.

what are pi bonds?

this is the weaker type of covalent bond that is created by the parallel overlap of atomic orbitals between the two atoms involved in the bond. pi bonds only exist in double and triple covalent bonds. in a double bond, one of the two bonds is/are a pi bond. in a triple bond, two of the three bonds is/are a pi bond.

can you have three pi bonds between two atoms?

no, because a triple bond is the maximum covalent bond type that has 1 sigma bond so it can only have 2 pi bonds.

why are pi bonds important in molecules?

sigma bonds allow the two atoms to rotate/spin in relation to one another. if pi bond(s) exist, they prevent this rotation and stabilize locations of the atoms within the molecule. a good way to remember it is that “S”igma bonds can spin while “P”i bonds prevent spinning.

what is bond order?

the number of bonding pairs of electrons between the atoms. it is used as a measure of the length/strength of a bond. if the covalent bond has more electron pairs shared between the atoms, the bond will be shorter. if the covalent bond has less electron pairs shared between the atoms the bond will be longer.

bond order length (longest —> shortest)

single, double, triple

what are resonance structures?

if a central atom has multiple of the same element covalently bonded but some are single bonded and some are double bonded, the electrons from the pi bond(s) will resonate between the atoms of the same type. this means the pi bonds will move back and forth between bonding locations. resonance can only occur between atoms of the same element bonded to the central atoms. when resonance conditions exist, you need to draw all of the possible resonance structures.