Lecture 2: Molecular orbitals, hybrid orbitals and geometry Flashcards
What do bonding and antibonding pi orbitals look like?
In the bonding pi orbitals, the electrons are above and below the central line between the two nuclei, so are shared between the two nuclei. This is favourable, but not as favourable as a σ (sigma) bond, which has the electrons directly between the two nuclei.
In anti-bonding pi orbitals, the electrons are not between the two nuclei, but on the other side of one nucleus from the other nucleus. This means the electrons are not being shared by the nuclei, which is necessary to form a covalent bond.
What are non-bonding orbitals?
Orbitals which do not overlap when two atoms bond together. This means that even in a molecule, these orbitals are effectively still atomic orbitals. Examples include 1s and 2s.
How are the 2p orbitals arranged?
What does the energy level diagram for molecular orbitals in the oxygen molecule look like?
The two unpaired electrons in the anti-bonding pi orbital mean that oxygen is very reactive and so must be handled carefully in cells. They also mean that oxygen is paramagnetic.
How do you calculate the bond order?
(e- in bonding orbitals - e- in anti-bonding orbitals) / 2
The 2s and three 2p orbitals of methane do not show the tetrahedral shape of the actual molecule. How is this resolved?
Hybridising the orbitals, to sp3, means that the correct shape is obtained. Hybridising is like averaging out all the energy so that the four orbitals are identical sp3 orbitals rather than one 2s and three 2 orbitals.
What hybridisation is a water molecule?
sp3. It has two sigma bonds to hydrogen atoms and two lone pairs of electrons, which each residue in an orbital.
How are the orbitals arranged in ethene?
The 2s orbital combines with two of the p orbitals, creating three hybrid sp2 orbitals which are planar (120 degrees bond angle). This leaving the third p orbital, the pz orbital, which sticks up above and down below the plane. This pz orbitals of two atoms can overlap, creating a pi bond. Combined with the sigma bonds from the hybridised orbitals, this creates a double bond.
Which is shorter, a single bond or a double bond? Give specific lengths and explain why this is the case.
A C=C double bond is shorter (1.34 A) than a C-C single bond (1.45 A). This is because the extra pi bond in the C=C double bond puts extra electrons between the nuclei, so the nuclei repel each other less, so can get closer together.
How many metres are there is a micrometre (um)?
10-6 metres
How many metres are there in a nanometre (nm)?
10-9 metres
Compare the strengths of C-C single and C=C double bonds.
A C=C double is not double the strength of a C-C single bond. The C-C single bond has about 350 kJ/mole of energy, whereas the C=C has about 600 kJ/mole of energy. This is because the sigma bond puts electrons directly between the nuclei, so is more favourable than the pi bondm which doesn’t put electrons directly between the nuclei.
