2.2. Molecular Orbitals

Question 1

What happens when atomic orbitals overlap end-on along an axis?

Answer 1

A sigma bond is formed.

Question 2

If two p orbitals overland end-on perpendicular to an axis what happens?

Answer 2

A pi bond is formed.

Question 3

What bonds are present in a double bond?

Answer 3

One sigma and one pi bond.

Question 4

What bonds are present in a triple bond?

Answer 4

One sigma and two pi bonds.

Question 5

Which are weaker, pi or sigma bonds?

Answer 5

Pi bonds as the side-on overlap is much less efficient.

Question 6

What is hybridisation?

Answer 6

The process of mixing atomic orbitals in an atom to create new orbitals called hybrid orbitals.

Question 7

What type of hybridisation does carbon have in alkanes?

Answer 7

sp3 hybridisation.

Question 8

What type of molecular bonding happens in alkanes, and what orbitals overlap?

Answer 8

1s orbitals of hydrogen overlap with the sp3 orbitals of the carbon creating sigma bonds.

Question 9

What type of molecular bonding happens in alkenes, and what orbitals overlap?

Answer 9

sp2 hybridisation happens between the 2s and two 2p orbitals to create three sp2 orbitals. These sp2 orbitals overlap with the 1s orbitals of the hydrogens to form sigma bonds. On each carbon there is still an unhybridised p orbital which overlaps side on with another unhybridised p orbital creating pi bonds.

Only the carbons involved in the double bond are sp2 hybridised.

Question 10

What do orbitals look like in non-polar covalent molecules?

Answer 10

They are symmetrical at the midpoint of the two atoms.

Question 11

What do orbitals look like in polar covalent molecules?

Answer 11

They have a larger part on the side of the atom with a slightly negative charge.

Question 12

What do orbitals look like in ionic molecules?

Answer 12

The orbitals only embrace the negative ion in the bond.

Question 13

What type of bonding happens in alkynes?

Answer 13

sp hybridisation occurs here when one 2s orbital and one 2p orbital hybridise. The remaining orbitals are left unhybridised. These two unhybridised p orbitals overlap side on with other unhybridised p orbitals and form pi bonds.

Question 14

Why are most organic compounds colourless?

Answer 14

They absorb wavelengths that aren’t in the visible spectrum.

Question 15

Give the molecular orbitals in order of increasing energy.

Answer 15

sigma bonding orbital

pi bonding orbital

sigma anti-bonding orbital

pi anti-bonding orbital

Question 16

What does HOMO stand for?

Answer 16

Highest Occupied Molecular Orbital

Question 17

What does LUMO stand for?

Answer 17

Lowest Unoccupied Molecular Orbital

Question 18

What molecular orbitals are the HOMO and LUMO in compounds with only C-C and no non bonding electrons?

Answer 18

HOMO - sigma bonding orbital

LUMO - sigma non-bonding orbital

Question 19

What are characteristics all compounds that give their items colour have?

Answer 19

A benzene ring.
A large number of carbons.
A large number of C=C.

Question 20

What do long conjugated structures allow for?

Answer 20

Delocalised electrons as the alternating sigma and pi bonds allow the transfer of electrons between them.

Question 21

What classifies a conjugated system?

Answer 21

A compound with alternating single and double bonds.

Question 22

What is the relationship between degree of conjugation and the difference in energy between the HOMO and the LUMO?

Answer 22

The higher the degree of conjugation, the smaller the gap between the HOMO and LUMO.

Question 23

What is a chromophore?

Answer 23

The group of atoms in a molecule that give it colour.