Lecture 30 Chapter 10 Sections 1-2

• Orbital overlap = bonding

• Molecular orbitals

• Hybrid orbitals

Announcements

• CAPA #17 due tomorrow• Seminar Friday 3:00• Seminar Tuesday 11:00

Bond Lengths and Energies

• Bond length – the nuclear separation distance where the molecule is most stable.1. The smaller the principle quantum numbers of the valence

orbitals, the shorter the bond.2. The higher the bond multiplicity, the shorter the bond.3. The higher the effective nuclear charge of the bonded atoms, the

shorter the bond.4. The larger the electronegativity difference, the shorter the bond.

• Bond energy – the stability of a chemical bond1. Bond strength increases as more electrons are shared between the

atoms2. Bond strength increases as the electronegativity difference (∆χ)

between bonded atoms increases.3. Bond strength decreases as bonds become longer.

Table 9 – 1: Average Bond Lengths

Orbital Overlap = Bond

As two hydrogen atoms approach each other, the overlap of their 1s atomic orbitals increases. The wave amplitudes add, generating anew orbital with high electron density between the nuclei.

This high electron density bonds the atoms together.

Molecular Orbitals• Bonding orbitals are constructed by combining atomic

orbitals from adjacent atoms.• So, two 1s atomic orbitals on two Hs yield two molecular

orbitals (MOs) in H2

Bonding Anti-bonding

Conventions of the Orbital Overlap Model

1. Each electron in a molecule is assigned a specific orbital.2. No two electrons in a molecule have identical

descriptions, because Pauli exclusion principle applies to electrons in molecules as well as in atoms.

3. The electrons in molecules obey the aufbau principle, meaning that they occupy the most stable orbitals available to them.

4. Even though every atom has an unlimited number of atomic orbitals, the valence orbitals are all that are needed to describe bonding.

Diatomic Molecules: HF and F2

Hybridization of atomic orbitals

• First described by Linus Pauling.• Most molecules have symmetric shapes that cannot be

described by s orbitals and/or p orbitals.• Atomic orbitals can be hybridized (combined) to generate

a new set of directional orbitals.• These mixed orbitals match the orbital geometry of the

compounds.

• All electrons around the central atom must still be in orbitals whether they are nonbonding electrons or bonding electrons.

s and p hybridization

Methane hybridization

• CH4 has tetrahedral structure…carbon is bonded to 4 hydrogen atoms.

• The s and all the p orbitals are needed for directional bonding, therefore, the s and the px, py, and pz hybridize.

• The new orbitals are called sp3.• Any inner atom with a steric number of 4 has tetrahedral

electron group geometry and can be described using sp3

hybrid orbitals.

General Features of Hybridization

1. The number of valence orbitals generated by the hybridization process equals the number of valence atomic orbitals participating in hybridization.

• Orbitals are modified – not created or destroyed2. The steric number of an inner atom uniquely determines

the number and type of hybrid orbitals.3. Hybrid orbitals form localized bonds by overlap with

atomic orbitals or with other hybrid orbitals.4. There is no need to hybridize orbitals on outer atoms,

because atoms do not have limiting geometries. Hydrogen always forms localized bonds with its 1s orbital. The bonds formed by all other outer atoms can be described using valence p orbitals.

Methanol

sp2 hybrid orbitals

• Mixes an s orbital with two p orbitals (s+p+p)• Required by central atoms with steric number of 3 (trigonal

planar electron group geometry)

sp hybrid orbitals

• Mixes an s orbital with a p orbital (s+p)• Required by central atoms with steric number of 2 (linear

electron group geometry)

What is the hybridization of the atom labeled 5 in acetic acid?

25%25%25%25% A. s

B. pC. sp2D. sp3

What is the hybridization of the atom labeled 1 in acetic acid?

25%25%25%25% A. s

B. pC. sp2D. sp3

What is the hybridization of the atom labeled 7 in acetic acid?

25%25%25%25% A. s

B. pC. sp2D. sp3

sp3d hybrid orbitals

• Mixes an s orbital with three p orbitals and a d orbital (s+p+p+p+d)

• Required by central atoms with steric number of 5 (trigonal bipyramidal electron group geometry)

sp3d2

• Mixes an s orbital with three p orbitals and two d orbitals (s+p+p+p+d+d)

• Required by central atoms with steric number of 6 (octahedral electron group geometry)

Table 10 – 1: Summary of Valence Orbital Hybridization

Today• Finish CAPA #17

Friday• Read Chapt 10• Seminar Friday 3:00• Seminar Tues 11:00• I’m gone on Monday – Pikaart filling in

Remember: You are done with the homework when you understand it!