Hybridization & Molecular Orbital Theory

Chem 1211, chapter 9

What is hybridization?

• the mixing of different types of orbitals in the valence shell to make a completely new set of degenerate orbitals; also called Valence Bond Theory

• the newly formed degenerate orbitals are called hybrid orbitals

What is a sigma bond?

• a sigma bond is formed when the interactive atomic orbital point along the axis connects the two bonding nuclei

• can either be standard atomic orbitals or hybrid orbitals

• formed by head-to-head overlap of orbitals ‘

• first type of bond made in an electron group; all bonds will have exactly 1 sigma bond

What is a pi bond?

• a pi bond is formed when the bonding atomic orbitals are parallel to each other and perpendicular to the axis connecting the two bonding nuclei

• formed by the side-to-side overlap of unhybridized p and d orbitals; pi bonds do NOT involve hybrid orbitals

• formed after sigma bonds are formed; only appear in double or triple bonds

What are the issues with Valence Bond Theory (Hybridization)?

• hard to explain the concept of resonance

• does not always effectively predict energy

• and bond length

Describe molecular orbital theory (MO)?

• deals better with molecular orbitals that are delocalized over the entire molecule (resonance bonds, unpaired electrons, etc.)

• in MO theory, the electrons belong to the whole molecule, so the orbitals also belong to the whole molecule; so, electrons are no longer “stuck” on one atom, but are shared (delocalized) among all electrons in a molecule

• orbitals are also delocalized across the entire molecule

Describe molecular orbitals

• hold two electrons, just like atomic orbitals

• fill the orbitals with the same rules

• indicate where electrons are likely to be found

• are not degenerate average like those in hybridization

What happens when we combine two atomic orbitals to form two molecular orbitals?

• we have one molecular orbital lower in energy (bonding MO)

• we have one molecular orbital higher in energy (anti bonding MO)

Describing Bonding MO

• when orbitals combine constructively, the resulting MO has less energy than the original atoms orbitals

• most of the electron density between the nuclei

Describe anti bonding MO

• when the orbitals combine destructively, the resulting MO has more energy than the original atomic orbitals

• no electron density between the nuclei

Describe bond order?

• bond order = # of bonding electrons - # of anti bonding electrons / 2

• we can use bond order to determine whether or not a bond will form: a negative or zero bond order indicates that a bond will NOT form

What are homonuclear diatomic molecules?

• defined as molecules with two of the same element

• to participate in molecular orbitals, atomic orbitals must overlap; if they don’t, we assume those electrons are localized

True or false: when two atoms bond, only the valence orbitals of the atoms contribute significantly to the molecular orbitals of a particular molecule

True

Describe homonuclear diatomic with s orbitals

• simple

• the valence s orbitals will overlap in the same fashion and forms one sigma bonding and one sigma anti bonding orbital

• s orbitals can only mix with other s orbitals in the same shell

Describe homonuclear diatomics with p orbitals

• when atoms have valence electrons in p orbitals, their molecular orbitals are slightly more complex

• valence p orbitals can overlap in several different ways: p orbitals can only mix with p orbitals in the same orientation

• → P_x orbitals: sigma bond (head to head overlap)

• → P_x & P_z orbitals: pi bond (side to side overlap)