Molecular Orbitals Flashcards
Where are electrons in molecules found?
Electrons in molecules are found in molecular orbitals. Each molecular orbital can hold 2 electrons which must have opposite spins.
How are molecular orbitals formed?
The number of molecular orbitals formed is equal to the number of atomic orbitals that combined to form it. If 2 atomic orbitals combined, 2 molecular orbitals will be formed.
What are the two types of molecular orbital?
1) Bonding molecular orbital - This orbital has a lower energy than the atomic orbitals that combined to make it. This orbital holds the nuclei, and the attraction between the positively charged nuclei and negatively charged electrons is the basis of bonding between atoms.
2) Antibonding molecular orbital - This orbital has a higher energy than bonding molecular orbital and the atomic orbitals that combined to make it.
How is a sigma bond formed?
Sigma bonds are formed when the atomic orbitals overlap along the axis of the bond (end-on overlap).
How is a pi bond formed?
A pi bond is formed when the atomic orbitals overlap parallel to the axis of the bond (side-on.
Why is a carbon to carbon double bond not twice as strong as a carbon to carbon single bond?
Because side-on overlap (pi bonds) is much less efficient than end-on overlap (sigma bonds). This means that pi bonds are weaker than sigma bonds.
What is hybridisation?
Hybridisation is the process of mixing atomic orbitals within an atom to form a new set of degenerate hybrid orbitals.
How does sp^3 hybridisation in Carbon happen?
One of the paired 2s electrons is promoted to to the empty 2p orbital to form four hybrid orbitals known as sp^3 hybrid orbitals.
How can bonding in alkanes be explained.
Bonding in alkanes can be explained in terms of sigma bonds and sp^3 hybridisation. For example, in ethane, the 4 sp^3 hybrid orbitals on each carbon atom will overlap end-on with 3 hydrogen 1s orbitals, and the other sp^3 hybrid orbital from the other carbon atom. Therefore, ethane has 7 sigma bonds.
How can bonding in alkenes be explained?
Bonding in alkenes can be explained in terms of sp^2 hybridisation, sigma bonds, and pi bonds.
How does sp^2 hybridisation in Carbon happen?
One of the electrons in the 2s orbital is promoted to the remaining 2p orbital to make two half-filled, stable subshells. The 2s orbital then mixes with two of the 2p orbitals to form three degenerate sp^2 hybrid orbitals. The remaining 2p orbital is left unhybridised.
How does bonding in ethene happen?
The sp^2 hybrid orbitals adopt a trigonal planar arrangement to minimise repulsion. Each carbon atom forms sigma bonds with two 1s hydrogen orbital and with one sp^2 hybrid orbital in the other carbon. The remaining unhybridised 2p orbitals left on the carbon atoms overlap side-on to make a pi bond. Therefore, ethene has 5 sigma bonds and one pi bond.
What is the bonding continum?
The bonding contium goes from non-polar covalent bonds to ionic bonds, with polar covalent bonds inbetween.
Give the definition for non-polar covalent bonding, covalent bonding, and ionic bonding.
Non-polar covalent - Where the atoms in the bond have the same electronegativity so the electrons are shared equally.
Polar covalent - Where the atoms in the bond have a small difference in electronegativity so the electrons are shared slightly unequally. The atom with the higher electronegativity will have a slightly negative charge and vice versa.
Ionic - Where the atoms have a large difference in electronegativity so the electrons have been transferred from one atom to the other.
Describe non-polar covalent, covalent bonding and ionic bonding in terms of molecular orbitals.
Non-polar covalent - The bonding molecular orbital is completely symemetrical about the midpoint between the two atoms of the bond.
Polar covalent - The bonding molecular orbital is asymmetrical about the midpoint between the two atoms as the orbital is pushed more towards the atom with the greater electronegativity.
Ionic - The bonding molecular orbital embraces only one atom (containing both electrons of the bond) of the bond so there is extreme asymmetry.
