3a- Molecular orbitals Flashcards
VSEPR cannot explain the bonding in all compounds. What can provide an explanation for more complex molecules?
Molecular orbital theory
When do molecular orbitals form?
When atomic orbitals combine
What is the number of molecular orbitals formed equal to?
The number of atomic orbitals that combine, e.g. 2 atomic orbitals combining produces 2 molecular orbitals
What does the combination of two atomic orbitals result in the formation of?
A bonding molecular orbital and an antibonding orbital
What does the bonding molecular orbital encompass?
Both nuclei
What is the basis of bonding between atoms?
The attraction of the positively charged nuclei and the negatively charged electrons
How many electrons can each molecular orbital hold?
A maximum of two
Out of non-polar covalent bonds, polar covalent bonds and ionic compounds which ones show the most to least symmetry?
Non-polar covalent bond = symmetrical
Polar covalent bond = not symmetrical
Ionic compounds = extreme asymmetry
What are sigma molecular orbitals or sigma bonds?
Covalent bond made of molecular orbitals made by end-on overlap of atomic orbitals
What are pi molecular orbitals or pi bonds?
Covalent bonds made of molecular orbitals made by side-on overlap of atomic orbitals
The electronic configuration of an isolated carbon atom cannot explain the number of bonds formed by carbon atoms in molecules. What can be used to explain the bonding and shape of molecules of carbon?
Hybridisation
What is hybridisation?
The process of mixing atomic orbitals within an atom to generate a ste of new atomic orbitals called hybrid orbitals
What can be said about these new hybrid orbitals?
They are degenerate
What type of hybridisation takes place in alkanes and what kind of arrangement do they adopt?
sp^3, they adopt a tetrahedral arrangement
What type of hybridisation takes place in alkenes and what kind of arrangement do they adopt?
sp^2, they adopt a trigonal planar arrangement
What type of hybridisation takes place in alkynes and what kind of arrangement do they adopt?
sp hybridisation, they adopt a linear arrangement
What can be used to explain why organic compounds are colourless or not?
Molecular orbital theory
What is the HOMO?
The highest bonding molecular orbital containing electrons
What is the LUMO?
The lowest antibonding molecular orbital
What can the absorption of electromagnetic energy cause?
Electrons to be promoted from HOMO to LUMO
Why are most organic molecules colourless?
Because the energy difference between HOMO and LUMO is relatively large, this results in absorption of light from the ultraviolet region of the spectrum
Some organic molecules contain chromophores. What is a chromophore?
A group of atoms within a molecule that is responsible for absorption of light in the visible region of the spectrum
When can visible light be absorbed?
When electrons in a chromophore are promoted from the HOMO to the LUMO
Chromophores exist in molecules containing what?
A conjugated system
What is a conjugated system?
A system of adjacent unhybridised p orbitals that overlap side-on to form a molecular orbital across a number of carbon atoms
What can be said about electrons within a conjugated system?
They are delocalised
What molecules have conjugated systems?
- molecules with alternating single and double bonds
- aromatic molecules
What is the relationship between the number of atoms in the conjugated system and the size of the energy gap between the HOMO and the LUMO?
The more atoms in the conjugated system the smaller the energy gap between HOMO and LUMO
When the wavelength of light absorbed is in the visible region, what colour will the compound exhibit?
The complementary colour
