7. Molecular Orbitals

what causes molecular orbitals to form

when atomic orbitals combine

what is the number of molecular orbitals formed equal to

the number of atomic orbitals that combine

what is made when theres a combination of 2 atomic orbitals

the formation of a bonding molecular orbital and an antibonding orbital

whats the difference between antibonding and bonding molecular orbitals

anti-bonding has higher energy than bonding

what does the bonding molecular orbital emcompass

both nuclei

how many electrons can a molecular orbital hold

2

what causes the attraction in the bonding between atoms

the attraction of the positively charged nuclei and the negatively charged electrons in the bonding molecular orbital

what shape does the molecular orbital have in a non polar covalent bond

symmetrical about the midpoint between the atoms

what shape does the molecular orbital have in a polar covalent bond

asymmetrical about the midpoint between the atoms

in polar covalent bonds which atom would have the greater share of bonding electrons

the atom with the greater value for electronegativity

what causes the formation of ions

the bonding molecular orbitals being almost entirely located around just 1 atom

how do σ bonds form

end-on overlap of atomic orbitals along the axis of the covalent bond

how do π bonds form

side-on overlap of parallel atomic orbitals that lie perpendicular to the axis of the covalent bond

which bond is stronger between σ and π and why

σ as end-on overlap is more efficient so are stronger

what is hybridisation

the process of mixing atomic orbitals within an atom to generate a set of new atomic orbitals called hybrid orbitals

what kind of energy do hybrid orbitals have

degenerate

what happens in the bonding of alkanes

the 2s orbital and the three 2p orbitals of carbon hybridise to form 4 degenerate sp³ hybrid orbitals adopting a tetrahedral arrangement

how can the bonding in alkanes be described

sp³ hybrid orbitals overlap end-on with other atomic orbitals to form σ bonds

what happens in the bonding of alkenes

the 2s orbital and two of the 2p orbitals hybridise to form 3 degenerate sp² hybrid orbitals adopting a trigonal planar arrangement

how can the bonding in alkenes be described

hybrid sp² orbitals overlap end-on to form σ bonds

the remaining 2p orbital on the carbons of the double bond are unhybridised and lie perpendicular to the axis of the σ bond. the unhybridised p orbitals overlap side-on to form π bonds

what happens in the bonding of alkynes

the 2s orbital and one 2p orbital of carbon hybridise to form 2 degenerate sp hybrid orbitals adopting a linear arrangement

how can the bonding in alkynes be described

the hybrid sp orbitals overlap end-on to form σ bonds

the remaining two 2p orbitals on each carbon atom lie perpendicular to each other and to the axis of the σ bond. the unhybridised p orbitals overlap side-on to form 2 π bonds

what happens in the bonding of benzene and other aromatic systems

the 6 carbon atoms in benzene are arranged in a cyclic structure with σ bonds between the carbon atoms causing sp² hybridisation

how can the bonding in benzene and other aromatic systems be described

the unhybridised p orbitals on each carbon atom overlap side-on to form a π molecular system, perpendicular to the plane of the σ bonds, extending across all 6 carbon atoms

what kind of electrons do benzene and other aromatic systems have

delocalised

what is molecular orbital theory used to explain

why organic molecules are colourless or coloured

how do electrons fill bonding molecular orbitals

from lowest energy to highest leaving the higher energy antibonding orbitals unfilled

what is the highest occupied molecular orbital (HOMO)

the highest bonding molecular orbital containing electrons

what is the lowest unoccupied molecular orbital (LUMO)

the lowest antibonding molecular orbital

what causes electrons to be promoted from HOMO to LUMO

absortption of electromagnetic energy

what causes most organic molecules to appear colourless

the energy difference between HOMO and LUMO is relatively large resulting in the absorption of light from the UV region

what can some organic molecules contain

chromophores

what is a chromophore

a group of atoms within a molecule thats responsible for absorption of light in the visible region of the spectrum

how can light be absorbed

when electrons in a chromophore are promoted from the HOMO to LUMO

what type of molecule do chromophores exist in

ones containing conjugated systems

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 kind of electrons do conjugated systems have

delocalised

what type of molecules have conjugated systems

molecules with alternating single and double bonds, and aromatic molecules

what happens when there are more atoms in the conjugated system

smaller energy gap between HOMO and LUMO and a lower frequency of light is absorbed by the compound

what colour will be exhibited if the wavelength of light is in the visible region

complementary

what does each line of a double bond represent

one represents end on overlap to form a σ bond

one represents side on overlap to form a π bond