CbD - Colour Flashcards
What is an electron configuration?
The arrangement of electrons in an atom.
What is electron configuration described in terms of?
Shells and sub-shells.
What happens when electrons move between shells?
Their energy levels change.
What happens in an atom when a covalent bond forms?
Atomic orbitals link up to form molecular orbitals.
Which molecular orbitals fill up first?
The lowest energy orbitals.
What are molecular orbitals?
Atomic orbitals link up to form molecular orbitals when a covalent bond is formed.
By absorbing exactly the right amount of energy what can electrons do?
Move from a filled molecular orbital to an empty one - forming an excited state.
What can the energy absorbed be in the form of?
UV or visible light.
What happens if the energy gap filled and the empty molecular orbitals correspond to frequencies of light?
The molecule appears coloured.
When does a molecule appear coloured in terms of energy levels?
If the energy gap filled and the empty molecular orbitals correspond to frequencies of light.
What does the colour of the molecule match?
The complement of the frequency it absorbs.
What does delocalisation affect?
The energy needed to excite electrons.
The energy needed to excite electrons will be affected by what?
Delocalisation.
When does a single covalent bond form?
When two atomic orbitals, each holding one electron, come together to form two molecular orbitals.
What needs to be absorbed to excite the electrons if there is a large energy gap (in a single bond)?
High-frequency UV.
Where do the electrons in a single covalent bond go in the molecular orbital?
Since each molecular orbital can hold two electrons, only one is filled.
What is the energy gap like between the filled and unfilled molecular orbital in a single covalent bond?
Very large.
How many molecular orbitals does a double covalent bond contain?
4
How many molecular orbitals does a single covalent bond contain?
2
What is the energy gap like between the highest filled molecular orbital and the lowest empty molecular orbital in a double covalent bond compared to the energy gap in a single covalent bond?
There is a smaller energy gap between the highest filled molecular orbital and the lowest empty molecular orbital in a double covalent bond.
What needs to be absorbed to excite the electrons if there is a small energy gap (in a double bond)?
Lower frequency UV.
In a delocalised system (like benzene), how many molecular orbitals are formed?
Many.
What is the energy gap like between orbitals in a delocalised system?
The orbitals are even closer in energy than in a double bond.
What needs to be absorbed to excite the electrons if there is a VERY small energy gap (in a delocalised system)?
VERY low frequency UV and visible light.
What happens as delocalisation increases?
More molecular orbitals form and are closer in energy.
If more molecular orbitals that are closer in energy form as delocalisation increases, what does this mean needs to happen for the electrons to be excited?
Less energy is needed to excite electrons and the frequency of light absorbed drops.
Why do functional groups that extend the delocalisation in chromophores cause the colour to change?
Because more molecular orbitals that are closer in energy form as delocalisation increases meaning less energy is needed to excite electrons and the frequency of light absorbed drops.
Do all delocalised systems involve benzene rings?
No
What happens when C=C double and C-C single bonds alternate?
Electrons become delocalised.
What is conjugation?
When C=C double and C-C single bonds alternate causing electrons to become delocalised.
Equation linking wave speed, wavelength and frequency
wave speed = wavelength x frequency
What is a conjugated system?
Alternating single and double bonds.
What does a conjugated system allow?
Delocalisation of electrons across many atoms in the molecule.
What can a conjugated system contain?
π (Pi) electrons from C=O and C=N bonds.
Can also include lone pairs on oxygen and nitrogen if they are aligned in the correct direction to allow overlap with the system.
Relationship between energy and level of conjugation
Lower in energy (smaller the energy gap) if more conjugated.
Why does a conjugated system produce colour?
A conjugated system decreases the energy gap between ground electron state and the excited state. So electrons in the molecule are allowed to absorb visible light frequencies.
What do the size of the energy gaps mean for the wavelengths and frequencies of light that are absorbed?
Smaller energy gaps = longer wavelengths ( and lower frequencies) of light being absorbed.
How many π (Pi) bonds must be in a conjugated system for it to absorb light in the range of the visible light spectrum?
5+ (7 is best).
