Molecular Orbitals Flashcards
When do molecular orbitals form?
When atomic orbitals combine
The number or molecular orbitals formed is equal to…
The number of atomic orbitals that combine.
The combination of 2 atomic orbitals forms?
An anti bonding orbital and a bonding molecular orbital
The bonding molecular orbital encompasses what?
Both nuclei
What is the basis of binding between 2 atoms
The attraction of the positively charged nuclei and the negatively charged electrons in the binding molecular orbital.
Non polar covalent bonds?
Bonding molecular orbital is symmetrical about the midpoint between 2 atoms.
Polar covalent bonds result from?
Bonding molecular orbitals that are asymmetric about the midpoint between 2 atoms
Which atoms in a polar covalent bond gets the greater share of electrons?
The atom with the greater value for EN
Ionic compounds?
Extreme case of asymmetry, with the bonding molecular orbitals being almost entirely located around just 1 atom, resulting in formation of ions.
Sigma bonds?
Form by end on overlap of atomic orbitals along the axis of the covalent bond.
Pi bonds
Form by side on overlap of parallel atomic orbitals that lie perpendicular to the axis of the covalent bond.
Hybridisation definition?
The process of mixing atomic orbitals within an atom to generate a set of new atomic orbitals called hybrid orbitals.
How can you describe hybrid orbitals?
Degenerate.
Hybridisation in alkanes?
2s orbital and 3x 2p orbitals of carbon hybridise forming 4 degenerate sp3 hybrid orbitals. These adopt a tetrahedral arrangement. The sp3 hybrid orbitals overlap end in with other atomic orbitals to form sigma bonds
Hybridisation in alkenes?
2s orbital and 2x 2p orbitals hybridise to form 3x degenerate sp2 hybrid orbitals. These adopt an trigonal planar arrangement. Hybrid sp2 orbitals overlap end on forming sigma bonds. Remaining 2p orbital on each C atom of the double bond is unhybridised and lies perpendicular to the axis of the sigma bond. Unhybridised p orbitals overlap side in forming pi bonds
Hybridisation in benzene and other aromatic compounds?
The 6 C atoms in benzene are arranged in cyclic structure with sigma bonds between the C atoms. The unhybridised p orbitals on each C atom overlap side on to form a pi molecular system, perpendicular to the plane of the sigma bonds. This system extends across all 6 C atoms.
How can you describe the electrons in a pi molecular system?
Delocalised.
Hybridisation in alkynes?
2s and 2p orbital hybridise to form 2 degenerate sp hybrid orbitals. These adopt a linear arrangement. The hybrid sp orbitals overlap end on to form sigma bonds. Remaining 2x 2p orbitals on each C atom lie perpendicular to each other and to the axis of the sigma bond. Unhybridised p orbitals overlap side on to form 2x pi bonds.
How do electrons fill orbitals?
Electrons fill bonding molecular orbitals, leaving higher energy antibonding orbitals unfilled.
What does HOMO stand for?
Highest bonding molecular orbital containing electrons is called the Highest Occupied Molecular Orbital
What does LUMO stand for?
Lowest anti bonding molecular orbital is called the Lowest Unoccupied Molecular Orbital.
Why are most organic molecules colourless?
Energy difference between HOMO and LUMO is relatively large. This results in absorption of light from the UV region of the spectrum.
What is a chromophore?
A group of molecules within a molecule that are responsible for absorption of light in the visible region of the spectrum.
When is light absorbed in a chromophore?
When electrons in a chromophore are promoted from the HOMO to the LUMO.
Where do chromophores exist?
In 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 C atoms.
How can you describe electrons in a conjugated system?
Delocalised.
What type of molecules have conjugated systems?
Molecules with alternating double and single bonds and aromatic molecules.
The more atoms in the conjugated system…
The smaller the energy gap between HOMO and LUMO. A lower frequency of light is absorbed by the compound.
When the wavelength of light absorbed is in the visible region, the compound will exhibit…
The complementary colour.
