Bond Theories

Lewis Structures Tell us about bonds in a molecule Do not tell us about the shape of the molecule

Valence Bond Theory Based on the quantum model Says that covalent bonds form when orbitals of different atoms overlap

Bonds: When Orbitals Overlap Types sigma (σ) – the ends of the orbitals overlap pi (π) – the sides of the orbitals overlap

Single Bond s sublevel sigma bond Chemistry textbook, p. 162

Single Bond p sublevel sigma bond Chemistry textbook, p. 162

Question Which type of bond forms first between covalently bonded atoms? Sigma Pi Depends on the atom

Double Bond sigma and pi bond Chemistry textbook, p. 162

Triple Bond sigma and 2 pi bonds Chemistry textbook, p. 162

Bonds pi bond double bond triple bond forms only after a sigma bond weaker than a sigma bond double bond stronger than a single bond triple bond stronger than a double bond

Resonance S O S O SO2 molecule The molecule does not oscillate between the two shapes. An average (1.5) of the 2 bond possibilities results.

Resonance O3 molecule O O

Resonance NO3 molecule − N O O N − O N −

Octet Exceptions Molecules with an odd number of valence electrons to share O − N O Example: NO2

Octet Exceptions F F − B − F Elements (such as Boron) that settle for less than 8 valence electrons F − B − F F Example: BF3

Extra electrons go to the d sublevel. Octet Exceptions Elements with more than 8 valence electrons Extra electrons go to the d sublevel. Example: SF6

The Need for Another Theory Valence bond theory predicts liquid oxygen (O2) to be colorless and nonmagnetic. However, it is actually pale blue and magnetic.

Molecular Orbital (MO) Theory Each atom’s orbitals are replaced by totally new orbitals when a molecule forms. Molecular orbitals are ranked in order of increasing energy.

Molecular Orbital (MO) Theory Electrons fill molecular orbitals from lowest to highest energy. Apply the same concept to molecules that has been applied to atoms.

Molecular Orbital (MO) Theory Unpaired electrons can cause paramagnetism. The lower energy molecular orbitals are bonding orbitals; the higher ones are antibonding orbitals.