4.2.4

# Colours of Ions (A2 Only)

Test yourself

## d Sub-Shell Splitting

The colour of transition metals ions depends on their ligands and their geometries.

### Splitting

• Without ligands, all of the d orbitals have the same energy.
• In the presence of ligands, the orbitals will split. Some of them gain energy, and some of them lose energy.
• This is shown in the following diagram.
• The difference in energy of the upper level and the lower level is given the symbol ΔE.

### Light absorption

• Electrons will occupy the lower energy orbitals first. This is called the ground electronic state of the ion.
• If an electron absorbs energy equal to the energy gap, it can move to occupy the higher energy orbitals. This is called an excited electronic state.
• Electrons will absorb frequencies of light that contain enough energy to jump the energy gap.
• This is shown pictorially on the next slide.

### Calculating the energy gap

• You can calculate the energy gap from the wavelength of absorbed light by using the following formula:
• $\Delta E = h\nu = \frac{hc}{\lambda}$
• ν is the frequency of light in hertz, h is planck’s constant (which will be given if you need it), c is the speed of light in ms-1 and λ is the wavelength of the light in metres.
• The energy gap depends on the metal ion, its oxidation state, its ligands, and also its coordination number.

## Colours of Transition Metal Ions

The colour of a transition metal ion depends on the colour of the light it absorbs.

### Absorption process

• When a transition metal ion is in light, it will absorb the frequencies which correspond to the d sub-shell energy gap.
• The rest of the frequencies will be reflected.
• You only see the reflected light.

### Absorption example

• Suppose you have a metal complex which absorbs red light.
• The red light is removed from the light you can see.
• You see the rest of the colours in the spectrum.
• So the complex appears blue.
• Metal ions that absorb red light do NOT appear red, because there’s no red light for you to see.

### Identifying metal ions

• Every transition metal ion will be a different colour with different ligands.
• But we can identify all the hexaaqua ions (the ones with six water ligands), and we also know the colours of some other specific ones.
• The ones you need to know are on the next slide.

### Iron(III) in solution

• On the previous slide, we gave you the colours of certain ions. While these colours are true, you might not always observe them.
• Iron(III) in solution usually appears yellow or orange if its concentrated.
• This is because hexaaqua iron(III) is quite acidic, and will lose protons to become Fe(H2O)5(OH-) (and this is yellow).
• If you’re asked what colour hexaaqua iron(III) is, you should say purple. Just be aware that if it's in solution, you’ll have a yellow solution.
• We’ll talk about the acidic properties in a later module.

## Spectroscopy

You can use the colour of ions to find their concentrations. This uses a technique called spectroscopy.

### Principles of spectroscopy

• You can shine white light through a coloured filter to remove everything but that colour of light.
• You can then let this light fall on a sample of a transition metal solution.
• The more light it absorbs, the higher the concentration of the solution.
• We can compare the amount of light absorbed to a calibration curve (this is explained on the next slide).

### Calibration curves

• When doing spectroscopy, we can’t calculate the concentration of a sample without comparing it to known concentrations.
• We first measure the absorbances of solutions whose concentrations we know. Then we plot these on a graph.
• This graph is called a calibration curve. An example is on the next slide.
• We then use the absorbance of the unknown sample to work out from the graph what its concentration is.