Molecular Orbitals Flashcards
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
The combination of two atomic orbitals results in the formation of what two types of orbital?
Bonding and antibonding. The bonding molecular orbital encompasses both nuclei.
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
In a non-polar covalent bond, the bonding molecular orbital is …what…about the midpoint between two atoms.
Symmetrical
Polar covalent bonds result from bonding molecular orbitals that are…what…about the midpoint between two atoms?
Asymmetric
Which atom has the greater share of bonding electrons?
The atom with the greater electronegativity value
What causes the formation of ions?
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 is a sigma bond?
Molecular orbitals that form by end-on overlap of atomic orbitals along the axis of the covalent bond
What is a pi bond?
Molecular orbitals that form by side-on overlap of parallel atomic orbitals that lie perpendicular to the axis of the covalent bond
How can the shape of molecules of carbon be explained?
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.
Describe bonding in alkanes.
In alkanes, the 2s orbital and the three 2p orbitals of carbon hybridise to form four degenerate sp3 hybrid orbitals. These adopt a tetrahedral arrangement. The sp3 hybrid orbitals overlap end-on with other atomic orbitals to form sigma bonds.
Describe bonding in alkenes.
The bonding in alkenes can be described in terms of sp2
hybridisation. The 2s orbital and two of the 2p orbitals hybridise to
form three degenerate sp2 hybrid orbitals. These adopt a trigonal
planar arrangement. The hybrid sp2 orbitals overlap end-on to form
sigma bonds. The remaining 2p orbital on each carbon atom of the
double bond is unhybridised and lies perpendicular to the axis of the
sigma bond. The unhybridised p orbitals overlap side-on to form pi
bonds.
Describe bonding in benzene.
The bonding in benzene and other aromatic systems can be
described in terms of sp2 hybridisation. The six carbon atoms in
benzene are arranged in a cyclic structure with sigma bonds between the
carbon atoms. The unhybridised p orbitals on each carbon atom
overlap side-on to form a pi molecular system, perpendicular to the
plane of the sigma bonds. This pi molecular system extends across all
six carbon atoms. The electrons in this system are delocalised.
Describe bonding in alkynes.
The bonding in alkynes can be described in terms of sp hybridisation.
The 2s orbital and one 2p orbital of carbon hybridise to form two
degenerate hybrid orbitals. These adopt a linear arrangement. The
hybrid sp orbitals overlap end-on to form sigma bonds. The remaining
two 2p orbitals on each carbon atom lie perpendicular to each other
and to the axis of the sigma bond. The unhybridised p orbitals overlap
side-on to form two pi bonds.
What can molecular orbital theory be used to explain?
Why molecules are coloured or colourless
What is the highest bonding molecular orbital containing electrons called?
Highest occupied molecular orbital (HOMO)
What is the lowest antibonding
molecular orbital called?
Lowest unoccupied molecular orbital
(LUMO)
What can cause electrons to be promoted from HOMO to LUMO?
Absorption of electromagnetic energy
Why do most organic molecules appear colourless?
The energy
difference between HOMO and LUMO is relatively large. This results
in absorption of light from the ultraviolet region of the spectrum.
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
In what molecules can chromophores exist?
Molecules containing 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
How do electrons behave within a conjugated system?
They are delocalised
What molecules have conjugated systems?
Molecules with alternating single and double bonds, and aromatic molecules
How does the number of atoms in a conjugated system correspond the energy gap between HOMO and LUMO?
The more atoms in the conjugated system the smaller the energy gap between HOMO and LUMO
When the wavelength of light absorbed by a conjugated system is from the visible region what is observed?
The complementary colour
