Molecular orbitals

VSEPR limitation

VSEPR cannot explain bonding in all compounds, molecular orbital theory can explain more complex molecules

Molecular orbitals formation

Form when atomic orbitals combine

Number of molecular orbitals

Equals the number of atomic orbitals that combine

Bonding and antibonding orbitals

Form from the combination of two atomic orbitals

Bonding molecular orbital

Encompasses both nuclei and holds up to two electrons

Basis of bonding between atoms

Attraction between positively charged nuclei and negatively charged electrons in the bonding molecular orbital

Non-polar covalent bond

Bonding molecular orbital is symmetrical about the midpoint between two atoms

Polar covalent bond

Bonding molecular orbital is asymmetric with electrons shared unequally

Electronegativity effect

The atom with higher electronegativity has a greater share of bonding electrons

Ionic bonding and molecular orbitals

Bonding molecular orbital is almost entirely around one atom

Sigma (σ) molecular orbitals

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

Pi (π) molecular orbitals

Form by side-on overlap of parallel atomic orbitals perpendicular to the axis of the covalent bond

Carbon bonding explanation

Requires hybridisation since isolated carbon atom configuration cannot explain bonding

Hybridisation

Mixing of atomic orbitals within an atom to form degenerate hybrid orbitals

Alkane hybridisation

Carbon’s 2s and three 2p orbitals form four degenerate sp³ hybrid orbitals in a tetrahedral arrangement

Alkane bonding

sp³ orbitals overlap end-on to form σ bonds

Alkene hybridisation

Carbon’s 2s and two 2p orbitals form three degenerate sp² hybrid orbitals in a trigonal planar arrangement

Alkene bonding

sp² orbitals form σ bonds and remaining unhybridised 2p orbitals overlap side-on to form π bonds

Benzene bonding

Six carbon atoms form σ bonds with sp² hybrid orbitals and π system formed from overlapping unhybridised p orbitals

Benzene π system

Extends across all six carbon atoms and electrons are delocalised

Alkyne hybridisation

Carbon’s 2s and one 2p orbital form two degenerate sp hybrid orbitals in a linear arrangement

Alkyne bonding

sp orbitals form σ bonds

two remaining unhybridised 2p orbitals form two π bonds via side-on overlap

Molecular orbital theory and colour

Explains colourlessness or colour of organic molecules by electron transitions between orbitals

HOMO

Highest Occupied Molecular Orbital containing electrons

LUMO

Lowest Unoccupied Molecular Orbital

Electron promotion

Absorption of electromagnetic energy promotes electrons from HOMO to LUMO

Colourless organic molecules

Have large energy gap between HOMO and LUMO

Chromophores

Groups of atoms responsible for absorption of visible light by promoting electrons from HOMO to LUMO

Conjugated system

System of adjacent unhybridised p orbitals overlapping side-on to form delocalised molecular orbitals

Examples of conjugated systems

Molecules with alternating single and double bonds

Conjugation and energy gap

More atoms in conjugated system = smaller HOMO–LUMO gap

Light absorbed in conjugated systems

Lower frequency (longer wavelength

Visible absorption and colour

If visible light is absorbed