2.1 Molecular Orbitals Flashcards
complete
What forms when atomic orbitals combine
Molecular orbitals
The number of molecular orbitals produced when atomic orbitals combine
Equal to the number of atomic orbitals that combine
The combination of two atomic orbitals results in what
- The formation of a bonding molecular orbital and an antibonding orbital
- The bonding orbital encompasses both nuclei
- The attraction of the positively charged nuclei and the negatively charged electrons in the bonding molecular orbital is the basis of bonding between atoms
How many electrons can a molecular orbital hold
Two
Symmetry of bonding molecular orbitals in different bonds
- Non polar covalent: the bonding orbital is symmetrical and about the midpoint between two atoms
- Polar covalent bonds result from bonding molecular orbitals that are asymmetrical about the midpoint between two atoms. Higher electronegativity = greater share of bonding electrons
- lonic compounds: bonding molecular orbital being almost entirely located around just one atom, resulting in the formation of ions
Sigma bonds
Form by end-on overlap of atomic orbitals along the axis of the covalent bond
Form by end-on overlap of atomic orbitals along the axis of the covalent bond
Sigma bond
Pi bond
Molecular orbitals that form from side on overlap of parallel atomic orbitals that lie perpendicular to rule axis of the covalent bond
Molecular orbitals that form from side on overlap of parallel atomic orbitals that lie perpendicular to rule axis of the covalent bond
Pi bond
Hybridisation
Process of mixing atomic orbitals within an atom to generate a set of new atomic called hybrid orbitals. The hybrid orbitals are degenerate
Process of mixing atomic orbitals within an atom to generate a set of new atomic called hybrid orbitals. The hybrid orbitals are degenerate
Hybridisation
Hybridisation in alkanes
- The 2s orbital and the three 2p orbitals of carbon hybridise to form four degenerate sp3 hybrid orbitals
- They adopt a tetrahedral arrangement
- The sp3 hybrid orbitals overlap end-on with other atomic orbitals to form sigma bonds
- The 2s orbital and the three 2p orbitals of carbon hybridise to form four degenerate sp3 hybrid orbitals
- They adopt a tetrahedral arrangement
- The sp3 hybrid orbitals overlap end-on with other atomic orbitals to form sigma bonds
Hybridisation in alkanes
- The 2s orbital and the three 2p orbitals of carbon hybridise to form four degenerate sp3 hybrid orbitals
- They adopt a tetrahedral arrangement
- The sp3 hybrid orbitals overlap end-on with other atomic orbitals to form sigma bonds
Hybridisation in alkanes
Hybridisation in alkenes
- The 2s orbital and two of the 2p orbitals hybridise to form degenerate sp2 hybrid orbitals
- They 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 sigma bond
- The unhybridised p orbitals overlap side-on to form pi bonds
- The 2s orbital and two of the 2p orbitals hybridise to form degenerate sp2 hybrid orbitals
- They 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 sigma bond
- The unhybridised p orbitals overlap side-on to form pi bonds
Hybridisation in alkenes
Hybridisation in benzene and other atomic groups
- 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
- 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
Hybridisation in benzene and other atomic groups
how can the bonding of the following be described:
- alkenes
- benzene and other aromatic systems
- alkynes
- alkenes: sp2 hybridisation
- benzene and other aromatic systems: sp2 hybridisation
- alkynes: sp hybridisation
hybridisation in alkynes
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
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
hybridisation in alkynes
HOMO
highest occupied molecular orbital
the highest bonding molecular orbital containing electrons
LUMO
lowest antibonding molecular orbital
lowest unoccupied molecular orbital
lowest antibonding molecular orbital
lowest unoccupied molecular orbital
LUMO
highest occupied molecular orbital
the highest bonding molecular orbital containing electrons
HOMO
why are some organic molecules colourless and some are coloured?
- Absorption of electromagnetic energy can cause electrons to be promoted from HOMO to LUMO
- Most organic molecules appear colourless because the energy difference between HOMO and LUMO is relatively large. This results in the absorption of light from the ultraviolet region of the spectrum
- Some organic molecules contain chromophores. A chromophore is a group of atoms within a molecule that is responsible for the absorption of light in the visible region of the spectrum. Light can be absorbed when electrons in a chromophore are promoted from the HOMO to the LUMO
- the more atoms in the system, the smaller the gap between HOMO and LUMO. A lower frequency of light (longer wavelength, lower energy) is absorbed by the compound. When the wavelength of light absorbed is in the visible region, the compound will exhibit the complementary colour
conjugated system
a system of adjacent unhybridised p orbitals that overlap side-on to form a molecular orbital across a number of carbon atoms
Electrons within this conjugated system are delocalised
Molecules with alternating single and double bonds, and aromatic molecules have conjugated systems
in what molecules do chromophores exist
molecules containing 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
Electrons within this conjugated system are delocalised
Molecules with alternating single and double bonds, and aromatic molecules have conjugated systems
what exist in conjugated systems
chromophores
chromphores
a group of atoms within a molecule that is responsible for absorption of light in the visible region of the spectrum
