(a) molecular orbitals Flashcards
Can VESPER explain the bonding in all compounds?
No
What, other than VESPER, can provide an explanation for complex molecules?
Molecular orbital theory
When do molecular orbitals form?
When atomic orbitals combine
What is the relationship between the number of molecular orbitals that are formed and the number of atomic orbitals which are combined?
They are equal
What does the combination of two atomic orbitals result in?
A bonding and an anti-bonding orbital
Where are the nuclei situated in molecular orbitals?
Both are found in the bonding molecular orbital.
What is the basis of the bonding between atoms?
The attraction between the positively charged nuclei and the negatively charged electrons.
What is the maximum number of electrons per molecular orbital?
2
What is the bonding molecular orbital in a non-polar covalent bond?
Symmetrical, about the mid-point between the 2 atoms.
What is the bonding molecular orbital in a polar covalent molecule?
Asymmetrical about the 2 midpoint between 2 atoms.
What atom has the greater share of the bonding electrons?
The atom with the greater electronegativity.
What is a case of extreme asymmetry?
Ionic compounds
Where are the bonding molecular orbitals in an ionic compound?
Almost entirely located around one atom.
What does extreme asymmetry result in?
The creation of ions.
What kind of atomic orbital overlap occurs in sigma bonds?
End-on
What kind of atomic overlap occurs in pi bonds?
Side-on
How can carbons bonding and shape be deciphered?
Hybridisation
What is hybridisation?
The process of mixing atomic orbitals within an atom to generate a set of new atomic orbitals called hybrid orbitals.
What is the energy said to be in hybrid orbitals?
Degenerate
In alkanes the 2s orbitals and the 2p orbitals of the carbon hybridise to form what?
Degenerate sp3 hybrid orbitals
What shape is adopted by the sp3 hybrid orbitals?
Tetrahedral
How do the sp3 molecules overlap with other atomic orbitals?
End-on
What does the end on overlap of sp3 molecules with other atomic orbitals produce?
sigma bonds.
What can the bonding of 2 alkenes be described as?
sp2 hybridisation
What do the 2s orbitals and 2p orbitals hybridise to form?
sp2 hybrid orbitals
What shape do sp2 hybrid orbitals adopt?
Trigonal planar
How do sp2 orbitals overlap and what do they form?
End-on to form sigma bonds
What happens to the remaining 2p orbital on each carbon?
Remains unhybridised and lies perpendicular to the axis of the pi bond.
How do the remaining 2p orbitals overlap and what do they form?
Side-on to form pi bonds
How can the bonding of benzene and other aromatic systems be described?
sp2 hybridisation
What bond is between the carbon atoms in a benzene ring?
Sigma and pi
What structure does a benzene ring have?
Cyclical
What can the electrons in a benzene ring be described as?
Delocalised
How can the bonding between alkynes be described?
sp hybridisation
(sp) What do the 2s orbital and 2p orbital hybridise to form?
Two degenerate hybrid orbitals.
What arrangement do the 2 degenerate hybrid orbitals adopt?
Linear
How do the hybrid sp molecules overlap and what do they form?
End-on to form sigma bonds.
What happens to the 2 remaining 2p orbitals (sp)?
The overlap side on to form 2 pi bonds.
How else can molecular orbital theory be used?
To explain why organic molecules are colourless or coloured.
What orbital do electrons fill up first?
Bonding molecular orbitals.
What orbital is left unfilled?
Anti-bonding molecular orbitals.
What is the highest bonding molecular orbital containing electrons called?
Highest Occupied Molecular Orbital (HOMO)
What is the lowest anti bonding molecular orbital called?
Lowest Unoccupied Molecular Orbital (LUMO)
An absorption of electromagnetic energy can cause what?
Electrons to be promoted from HOMO to LUMO.
Why do most organic molecules appear colourless?
The energy difference between HOMO and LUMO is relatively large.
What does a large difference between HOMO and LUMO result in?
The absorption of light from the ultraviolet region of the spectrum.
What do 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.
