Molecular Orbitals

Hydrogen (H₂) Molecular Orbital

This sigma bond is formed by the overlap of two 1s atomic orbitals

The combination of a sigma and pi bond is

less than twice as strong as a single sigma bond

Molecules have...

molecular orbitals

Triple bond in nitrogen consists of

one sigma and two pi bonds 6 electrons in three bonding molecular orbitals each nitrogen atom has a lone pair which exists in a non-bonding molecular orbital

Order of bond lenghts

single bond > double bond > triple bond

Quantum Mechanics Bonding Molecular Orbitals are constructed by...

the addition of the 1s wave function for one hydrogen atom to the other

Bond length

the distance between the nuclei of two covalently bonded atoms

the shorter the bond length

the greater the attraction of both nuclei by the increased electron density between them

Bonding orbitals lead to..

the nuclei being held together

Molecular orbitals are formed by

the overlap of atomic orbitals

Sigma(σ) and pi(π) bonds result from

the overlap of atomic orbitals

Quantum Mechanics Antibonding Molecular Orbitals are constructed by...

the subtraction of the 1s wave function for one hydrogen atom from the other

When alkenes react with electrophilic addition

the weaker pi bond breaks during hte reaction

Bond Order tells

us whether a molecule has single bonding order = 1, double bonding order = 2

Electron density in Hydrogen Molecular Orbital

- DECREASE in electron density between nuclei in ANTIBONDING MO which INCREASES the electrostatic repulsion between the 2 nuclei - INCREASE in electron density between 2 nuclei in BONDING MO which DECREASES the electrostatic repulsion between the 2 nuclei - Means hydrogen molecule has the effective nuclear chargw e

Homolytic Bond Fission

- Irradiation of a hydrogen (chlorine) molecule by UV light - causes the atom to separate - as promotion of one electron from bonding molecular orbital to antibonding molecular orbital - 1/2(Nb-Na) = 1/2(1-1) = O - Hydrogen molecule flies apart cause there are no bonds

Weaker pi bonds are formwd by

- sideways on overlap of p oritals - pi bond in oxygen is formed by the overlap of two 2pz orbitals which are laterally oriented - double overlap causes a molecular orbital with electron density above and below the sigma bond - no negative charge along molecular axis

Molecular orbitals can hold up to

2 electrons with opposing (anti-parallel) spins

Electron density decreases between

2 nuclei in anti-bonding orbitals

Electron density increases between

2 nuclei in bonding molecular orbitals

Bond order formula

BO = 1/2 (Nb-Na)

Why is it harder to break a C=O double bond than a C-O single bond?

Because the increased electron density between the carbon and oxygen atom leads to a greater force of attraction between their nuclei. As Bond order doubles the bond energy more than doubles.

He₂ Molecular Orbital

Bond Order = 1/2 (Nb-Na) = 1/2(2 - 2) = 0

Hydrogen Molecular Orbital

Bond order = 1/2(Nb-Na) = 1/2(2-0) = 1/2(2) = 1 Single Bond Hydrogen

Carbon Monoxide (CO) Molecular Orbital

Carbon = 1s²2s²2p² Oxygen = 1s²2s²2p⁴ BO = 1/2(Nb-Na) = 1/2(8-2) = 3

Pi(π) bonds

Double and triple bonds Weaker than sigma bonds

Aufbau Principle

Ground state electrons go into the available orbital with lowest energy

Pauli's exclusion Principle

Only two electrons of opposite spin can be accommodated in an orbital

Hund's Rule

Orbitals in subshells having the same energy are occupied singly before being paired

Oxygen (O₂) Molecular Orbital

Oxygen = 1s²2s²2p⁴ BO = 1/2(Nb - Na) = 1/2(8-4) = 2 Double Bond

HCl Molecular orbital sigma

This sigma bond is formed by the overlap of 1s atomic orbital in hydrogen with the 3px orbital in chlorine

Sigma(σ) bonds

Single bonds Symmetrical Stronger than Pi Bonds

Chlorine (Cl₂) Molecular orbital sigma

This sigma bond can be formed by the 'head on' overlap of 3px orbitals from each of the chlorine atoms. This occurs when 2 3px orbitals are linearly oriented.

Can you have sigma bonds without pi orbitals?

Yes, but pi orbitals only form where there is already a sigma bond.

The strength of a covalent bond is..

a measure of how much lower in energy the bonding molecular orbital is compared with the atomic orbitals

Double bonds of alkenes contain

a sigma and pi bond

Excited state are

anti bonding orbitals

Molecular orbitals

are where electrons occupy in orbitals in covalent molecules

Why is the antibonding orbital (higher energy) not normally occupied?

as electrons in these types of orbitals pull the nuclei and hence the molecule apart

Atoms have...

atomic orbitals

Why are sigma bonds relatively stronger than pi bonds

because pi orbitals are less effective at holding atoms together than sigma orbitals

How are molecular orbitals different from atomic orbitals

because the electrons filling them are under the influence of two or more nuclei rather than just one nucleus.

3 types of molecular orbitals

bonding (ground state) antibonding (excited state) non bonding (lone pairs

Ground state are

bonding orbitals

Molecular orbital diagrams can be used to...

calculate bond order

The bonding orbital is where

electrons are found in the ground state for hydrogen molecules.

Antibonding orbitals (σ*) are...

higher in energy than bonding orbitals