Unit 3-Molecular Orbitals Flashcards
How are molecular orbitals formed?
When atoms approach each other, their separate sets of atomic orbitals merge to form a single set of molecular orbitals.
What is hybridisation?
Hybridisation is the process of mixing atomic orbitals within an atom to generate a set of new atomic orbitals called hybrid orbitals.
Describe sp3 hybridisation?
An s-orbital combines with 3 p-orbitals to form four sp3 hybrid molecular bonding orbitals of equal energy (degenerate).
Describe sp2 hybridisation?
An s-orbital combines with 2 p-orbitals to form three sp2 hybrid molecular bonding orbitals.
This will leave the third 2p orbital un-hybridised.
Describe sp hybridisation?
An s-orbital combines with 1 p-orbital to form two sp hybrid molecular bonding orbitals.
This will leave two 2p orbitals un-hybridised.
Which type of bond is created from the end-on overlap of two atomic orbitals lying along the axis of a bond?
Sigma (σ) bond
Which type of bond is created from the side on overlap of two atomic orbitals lying perpendicular to the bond?
Pi (π) bond
Describe how to count the number of σ and π bonds in a structure.
σ bonds are the first bond that an atom makes. Any additional bonds (double or triple) will be π bonds.
The number of sigma and pi bonds in a molecule of pyridine is
11 sigma and 3 pi
The number of sigma and pi bonds in the structure shown is?
Number of σ-bonds = 18
Number of π-bonds = 2
Explain why most organic molecules appear colourless?
The energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) is relatively large resulting in the absorption of light in the ultraviolet region of the spectrum.
Coloured organic compounds contain delocalised electrons within molecular orbitals which extend across several atoms.
What name is given to this system?
Conjugated system
How can a conjugated system be identified from a structural formula?
The structure contains alternating single and double bonds
What is the connection between the size of conjugation and the energy gap between HOMO and LUMO?
The more atoms the delocalised molecular orbital spans, the smaller the energy gap between the delocalised orbital and the next unoccupied orbital.
REMEMBER that the lower the band gap, the higher the wavelength of light that is absorbed.
What word is used to describe the part of the molecule responsible for absorbing colour (the conjugated part)?
Chromophore
