Molecular Orbitals Flashcards
What can VSEPR not explain
VSEPR cannot explain the bonding in all compounds.
What can provide an explanation for more complex molecules
Molecular orbital theory can provide an explanation for more complex molecules
When do molecular orbitals form
Molecular orbitals form when atomic orbitals combine
What is the number of molecular orbitals formed equal to
The number of molecular orbitals formed is equal to the number of atomic orbitals that combine
What does the combination of two atomic orbitals result in
The combination of two atomic orbitals results in the formation of a bonding molecular orbital (HOMO) and an anti bonding orbital (LUMO)
What can each molecular orbital hold
Each molecular orbital can hold a maximum of two electrons
What does the bonding molecular orbital encompass
The bonding molecular encompasses both nuclei
Which orbital bonding or anti bonding has the lowest energy
The bonding orbital always has the lowest energy
What is the basis of bonding between atoms
The attraction of the positively charged nuclei and the negatively charged electrons in the bonding molecular orbital is the basis of bonding between atoms
What is the arrangement of the bonding molecular orbital in a non - polar covalent bond
In a non-polar covalent bond, the bonding molecular orbital is symmetrical about the midpoint between two atoms.
What do polar covalent bonds result from
Polar covalent bonds result from bonding molecular orbitals that are asymmetric about the midpoint between two atoms.
Which atom has greater share of the bonding electrons
The atom with the greater value for electronegativity has the greater share of the bonding electrons
What are ionic compounds
Ionic compounds are an extreme case of asymmetry with the bonding molecular orbitals being almost entirely located around just one atom resulting in the formation of ions
What are sigma bonds
Molecular orbitals that form by end-on overlap of atomic orbitals along the axis of the covalent bond are called sigma (σ) molecular orbitals or sigma bonds
What are pi bonds
Molecular orbitals that form by side - on overlap of parallel atomic orbitals that lie perpendicular to the axis of the covalent bond are called pi (π) molecular orbitals or pi bonds
What can the electronic configuration of an isolated carbon not explain
The electronic configuration of an isolated carbon atom cannot explain the number of bonds formed by carbon atoms in molecules
How can the bonding and shape of molecules be explained
The bonding and shape of molecules of carbon can be explained by hybridisation
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. These hybrid orbitals are degenerate
What Do the 2s and 2p orbitals hybridise to form in alkanes
In alkanes, the 2s orbital and the three 2p orbitals of carbon hybridise to form four degenerate sp3 hybrid orbitals.
What arrangement do the orbitals in alkanes have
These adopt a tetrahedral arrangement
What bonds do the sp3 in alkanes form
The sp3 hybrid orbitals overlap end-on with other atomic orbitals to form σ bonds
What does hybridisation of alkanes produce
Hybridisation produces four identical orbitals which are a mixture of one s orbital and three p orbitals forming four new sp3 orbitals
In What terms can bonding in alkenes be explained
The bonding in alkenes can be described in terms of sp2 hybridisation
What do alkenes hybridise to form
The 2s orbital and two of the 2p orbitals hybridise to form three degenerate sp2 hybrid orbitals
What arrangement do the orbitals in alkenes have
These adopt a trigonal planar arrangement
What bonds do the Sp2 orbitals and remaining 2p orbitals form
The hybrid sp2 orbitals overlap end -on to form sigma bonds. The remaining 2p orbital on each carbon atom of the double bond is un hybridised and lies perpendicular to the x-axis of the sigma bond. The un hybridised p orbitals overlap side - on to form pi bonds
Alkynes
In the excited state one of the s electrons is promoted to the p shell, the s orbital and one of the p orbitals then hybridise to sp hybrid orbitals
Which bond do alkenes have
This means that the c-c double bond one sigma and one pi bond
What bonds do alkynes form
This molecule contains a triple bond formed from two pi bonds and one sigma bond
When is energy absorbed
Energy (and therefore light) is absorbed when electrons are promoted from the HOMO to the LUMO
Why are most organic compounds colourless
Most organic compounds are colourless as the energy gap between the HOMO and the LUMO molecular orbitals is relatively large. This results in absorption of light from the ultraviolet region of the spectrum
What do some organic compounds contain
Some organic compounds contain chromophores
What is a chromophore
A chromophore is a group of atoms within a molecule responsible for absorption of light in the visible region of the spectrum and therefore responsible for its colour
Which molecules do chromophores exists in
Chromophores exist in molecules containing a conjugated system
What is a conjugated system
A system of connected p -orbitals containing delocalised electrons over several atoms, which lowers the overall energy of the molecule and increases stability
Which molecules have conjugates systems
Molecules with alternating c=c bonds and c-c and aromatic molecules
What does the conjugated system begin and end with
As the conjugated system is a system of connected p orbitals it begins and ends with a c-c bond
What do the conjugated double bonds allow
The conjugated double bonds reduce the energy gap between the HOMO and the LUMO molecular orbitals allowing the compound to absorb in the visible part of the spectrum
What does the wavelength of light absorbed depend on
The wavelength of light absorbed depends on the length of the conjugated chain
The larger the conjugated system…
The larger the conjugated system the lower the energy of light absorbed and therefore the longer the wavelength of light absorbed
Where is the wavelength of molecules which contain chromophores
In molecules which contain chromophores that wavelength is in the visible region of the spectrum
When does the colour arise
The colour arises as the electrons absorb energy and are promoted from the HOMO to the LUMO. The colour of the compound is the colour complementary to that absorbed
