2.3 molecular orbitals Flashcards
when do molecular orbitals form?
what atomic orbitals combine
the number of molecular orbitals formed…
is equal to the number of atomic orbitals that combine
the combination of two atomic orbitals results in the formation of…
a bonding molecular orbital (HOMO) and an antibonding molecular orbital (LUMO)
the bonding molecular orbital encompasses…
both nuclei
describe the attraction that is the basis of bonding between atoms
positively charged nuclei and negatively charged electrons in the bonding molecular orbital
how many electrons can each molecular orbital hold?
a maximum of 2
describe the bonding molecular orbital in a non-polar covalent bond
symmetrical about the midpoint between 2 atoms
describe the bonding molecular orbital in a polar covalent bond
asymmetric about the midpoint between two atoms
in a polar covalent bond, which atom has a greater share for bonding electrons?
the atom with the highest electronegativity
where are bonding molecular orbitals located in an ionic bond?
almost entirely around just one atom
describe sigma (σ ) bonds
bonds that form when molecular orbitals form by end-on overlap of atomic orbitals
describe pi (π ) bonds
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 bonding and shape of orbitals be explained?
hybridisation
what is hybridisation?
the process of mixing atomic orbitals within an atom to generate a set of new atomic orbitals called hybrid orbitals
hybrid orbitals are…
degenerate
which arrangement do the four sp3 orbitals adopt in alkanes?
tetrahedral
how many sigma bonds are in ethane?
7
which arrangement do the three sp2 orbitals adopt in alkenes?
trigonal planar
how do electrons fill MOs?
they fill the lower energy bonding ones first and leave the higher energy antibonding ones unfilled
where does end-on overlap occur?
along the axis of the bond that forms
why can sp3 hybridisation occur?
as 2s and 2p orbitals are close in energy, allowing one 2s orbital to be promoted to the 2p orbital
how many electrons occupy each sp3 orbital?
1
which is weaker, pi or sigma bonds?
pi
what is different about hybridisation in alkenes?
only two of the p orbitals mix with the 2s orbital
where does the unhybridised p orbital lie in alkenes?
perpendicular to the sp2 orbitals
what does a C=C bond consist of?
one sigma bond and one pi bond
how is a pi bond formed in alkenes?
sideways overlap occurs between the two unhybridised p orbitals
which type of hybridisation do carbon atoms in benzene undergo?
sp2
what occurs between the unhybridised p orbitals in aromatics?
a doughnut-shaped pi-bonding system above and below the carbon ring
how do the 6 electrons behave in a benzene pi system?
they are delocalised over the system
which type of hybridisation occurs in alkynes?
sp
how does sp hybridisation occur?
an electron from the 2s is promoted to the empty 2p. the 2s orbital and one of the 2p orbitals hybridize to form two degenerate sp hybrid orbitals
which shape do sp orbitals form around the carbon?
linear
where do the unhybridised p orbitals lie around the sp orbitals?
perpendicular to the hybrid and each other
which bonds does a carbon-carbon triple bond contain?
1 sigma and 2 pi bonds
the absorption of energy (corresponding to the energy gap) can cause…
electrons to be promoted from the HOMO to the LUMO
why does light absorbed in most organic molecules lie outwith the visible spectrum?
due to large energy gap
what is a chromophore?
a group of atoms within a molecule that is responsible for the absorption of light in the visible region of the spectrum
where do chromophores exist?
in molecules with a conjugated system
what is a conjugated system?
a system of adjacent unhybridised p orbitals that overlap sideways to form a molecular orbital across a number of carbon atoms where the electrons are delocalised
what does a conjugated system often consist of?
alternating single and double bonds (or multiple connected benzene rings)
a chromophore/conjugated system always…
starts and ends with a double bond
as the conjugated system gets larger…
the energy gap between the HOMO and the LUMO decreases
what does a decreased energy gap mean?
that lower energy/higher wavelength of light is required to promote an electron. light will be absorbed in the visible spectrum
different colours of organic compounds are the result of…
differing sizes of conjugated system
