Molecular Orbitals Flashcards
What are molecular orbitals
When atomic orbitals overlap, they combine to form molecular orbitals. This molecular orbital is more stable than each of the separate atomic orbitals and contributes to the shape of this molecule.
How does the bonding in molecular orbitals work?
Molecular orbitals form when 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 and an antibonding molecular orbital. The bonding molecular 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. Each molecular orbital can hold a max of two electrons.
Bonding( course spec)
In a non- polar covalent bond, the bonding molecular orbitals symmetrical about the midpoint between two atoms. The atom with the greater electronegativity value has the greater share of bonding electrons. Ionic compounds are an extreme case of asymmetry, with the bonding molecular orbitals being almost entirely located around just one atom, resulting in the formation of ions.
What are sigma bonds?
A covalent bond is formed when 2 half filled atomic orbitals overlap. If they overlap along the axis of the bond (end on) a covalent bond is known as a sigma bond.
What are pi bonds?
Molecular orbitals that form side-on overlap of parallel atomic orbitals that lie perpendicular to the axis of the covalent bond are called pi molecular orbitals or pi bonds.
The electronic configuration of an isolated carbon atom cannot explain the number of bonds formed by carbon atoms in molecules. The bonding and shape of molecules can be explained by hybridisation. What is hybridisation?
Hybridisation is the process of mixing atomic orbitals within an atom to generate a set of new atomic orbitals called hybrid orbitals. These hybrid orbitals are degenerate. Hybrid theory suggests that the 2s and 2p orbitals of carbon combine (or mix) to form 4 degenerate orbitals.
Sp3 hybrid orbitals
The 4 hybrid orbitals are arranged tetrahedrally around the nucleus (this minimises electron repulsion) For simplicity, the small lobe of the orbital is omitted. The hybrid orbital is better suited to bonding, than a p orbital- it is strongly directed in the bonding direction.
Hybridisation in ALKANES
In alkanes, the 2s orbital and the three 2p orbitals of carbon hybridise to form four degenerate sp3 hybrid orbitals. These adopt a tetrahedral arrangement. The sp3 hybrid orbitals overlap end on with other atomic orbitals to form sigma bonds.
Hybridisation in ALKENES
Hybridisation in alkenes gives 3 identical hybrid orbitals. However, this time each c atom used the 2s orbital and only 2 of the 2p orbitals, leaving one 2p orbital unhybridised.
In alkenes, the 3 sp2 orbitals repel each other, resulting in a bond angle of 120 degrees between them. The unhybridised p orbitals are perpendicular to the plane of the molecule. The p orbitals of the carbon atoms are parallel and close enough to overlap sideways.
This sideways overlap between the 2p orbitals produces a new molecular orbital between the 2 carbon atoms. A pi bond is a covalent bond formed by the sideways overlap of 2 parallel atomic orbitals. Pi bonds arise when atoms make multiple bonds, for example a double bond.
Hybridisation in ALKYNES
Alkynes- sp hybridisation
The 2s orbital and one 2p orbital of carbon hybridise to form 2 degenerate sp 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 the axis of the sigma bond. The unhybridised p orbitals overlap side on to form 2 pi bonds.
Sigma and pi bonds
Looking at information comparing sigma and pi bonds, we can see that the double bonds are stronger than single bonds, but not twice as strong. This is because the sideways overlap( pi bond) is weaker than the end on overlap ( sigma bond)
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C-C = 1 sigma orbital
C—C( double bond) 1 pi and 1 sigma
C—-C (triple bond) 1 sigma and 2 pi
Molecules and colour
Molecular orbital theory can be used to explain why organic molecules are colourless or coloured.
Organic compounds also absorb electromagnetic energy. Most are colourless because the wavelength of light absorbed does not lie within the visible range of EM spectrum. However there are some organic compounds which are coloured.
Electrons fill bonding molecular orbitals, leaving higher energy anti bonding orbitals unfilled.
⚪️The highest bonding molecular orbital containing electrons is called the Highest Occupied Molecular Orbital( HOMO)
⚪️The lowest anti-bonding molecular orbital is called the Lowest Unoccupied Molecular Orbital.
ORGANIC COMPOUNDS THAT CONTAIN ONLY SIGMA BONDS AND SOME PI ARE COLOURLESS.
THE SIGMA BONDING ORBITAL IS THE HOMO AND THE LUMO IS THE SIGMA ANTI-BONDING ORBITAL
Conjugated System
Organic molecules that are coloured contain delocalised electrons spread over a number of atoms, these molecules are known as a conjugated system.
