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