The colour of compounds

Question 1

What does HOMO stand for?

Answer 1

Highest Occupied Molecular Orbital

Question 2

What does LUMO stand for?

Answer 2

Lowest Unoccupied Molecular Orbital

Question 3

Why are many organic molecules like ethene colourless?

Answer 3

there is a large energy difference between HOMO and LUMO. Therefore, if an electron is to be promoted between these obritals absorption of light in the UV region is required.

Question 4

Why is buta-1,3-diene coloured?

Answer 4

It has a smaller energy gap between the orbitals. Therefore, absorption of light in the visible spectrum is required to promote electrons. As a result, it will appear coloured depending on the wavelength of light being absorbed and transmitted.

Question 5

What do coloured organic molecules contain?

Answer 5

delocalised electrons within molecular orbitals which extend over several atoms. The more atoms the decolalised molecular orbital spans, the smaller the energy gap between HOMO and LUMO is.

Question 6

What is a chromophore?

Answer 6

A group of atoms within a molecule that is responsible for its colour.

Question 7

If the chromophore absorbs light of one colour, what will the compound exhibit?

Answer 7

the complementary colour