Molecular Orbital Theory

Molecular Orbital Theory

(What is it??)

Better bonding model than valence bond theory

Electrons are arranged in “molecular orbitals”

Dealing with valence shell electrons

Molecular Orbitals

Combination of atomic orbitals 2 atomic orbitals------2 molecular orbitals

Molecular region where electrons are likely to be found within a chemical compound

Behave like molecules

Deals with electrons arranged in molecules, NOT atoms

Bonding Molecular Orbital (σ)

Contain electrons involved in chemical bonding Contribute to bond strength Increase stability

Lower energy level for electrons

Increased electron density between atoms

Anti-bonding Molecular Orbitals (σ*)

Contain electrons NOT involved in bonding Electrons hang out away from bond Decreased stability and bond strength

Higher energy level for electrons

Decreased electron density between atoms

**Electrons want to be at a LOW energy level SO---generally pair up and reside in bonding molecular orbitals.

How are electrons placed in molecular

orbitals? 1) Electrons want to be in the lowest-energy

molecular orbitals as possible.

2) Only 2 electrons found in each molecular orbital.

3) Electrons are placed in molecular orbitals by themselves (parallel spins) unless they have to be paired up (opposite spins).

Bond Order

Determined by molecular orbitals

= (# electrons in bonding MO) – (# electrons in anti-bonding MO)

2

Example 2: Molecular orbital energy-level

diagram

H2+

What happens when “p” atomic orbitals

combine? Each “p” orbital combines with another “p” orbital—2

molecular orbitals produced

Of the 2 molecular “p” orbitals— 1 lower energy bonding orbital 1 higher energy anti-bonding orbital

One p orbital produces orbital overlap σp , σ*p

Other 2 p orbitals overlap in parallel πp , π*p

Example 3: O2

Homework

Read pp. 417-418

Problems #61, 63, 67

Read over Thursday lab procedure