The material below was in a previous topic nad moved to this topic. The page looks at how the periodic table can be used in the context of transition metal colours. The core subject knowledge required may be covered in another course  and if so, can be linked to it there.

The post-16 atomic model can be used to explain the colourful nature of transition metal compounds. Transition metals are metals in the d-block that have partially filled d-orbitals.

Transition metals, in solution, are known to be colourful. When a transition metal is in an octahedral complex ie. is bonded to six other species, the d-orbitals are no longer degegenerate and are split into two groups with different energies.Three of the d-orbtials are lowered in energy and two of the d-orbitals are raised in energy.  The d-orbital electrons are distributed over the two groups according to the number of electrons and the size of the energy gap. 

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This feature of the d-orbitals gives rise to the colourful compounds. 

Examining copper compounds

Anhydrous copper(II) sulfate is white but hydrated copper(II) sulfate solution is blue. 

Look at the four images below and see if you can come up with a annotation or descriptions for each image to explain the colour change. 

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Image 1

Copper(II) has the electron configuration [Ar] 3d⁹. When in the solid form, all the 3d-orbitals have the same energy

Image 2

When the copper(II) sulfate is hydrated, a copper(II) hexaaqua complex is formed, [Cu(H₂O)₆]²⁺. The water ligands form an octahedral complex which splits the 3d-orbitals into two groups. The three lower energy d-orbitals contain 6 electron and the two higher energy d-orbitals contain 3 electrons. 

Image 3

When white light passes through the solution, an electron in on of the lower energy d-orbitals can be promoted to the higher energy d-orbitals through absoprtion of a photon of light. The energy of the absorbed photon (hv) matches the energy gap (ΔE) between the two groups of d-orbtials.

Note: h = Planck's constant, v = frequency of light

Image 4

The frequency of light absorbed by the electron corresponds to yellow/orange light. This colour of light is removed from the white light passing through the solution. As a result, the solution colour looks blue/cyan. Complementary colours on a colour wheel tell us the colour produced when one colour is removed from white light. Complementary colours are opposite each other on a colour wheel. 

Zinc is a d-block element with the electron configuration [Ar] 4s² 3d¹⁰. It forms the ion Zn²⁺. 

Why are zinc solutions colourless rather than colourful?

A Zn²⁺ ion has the electron configuration [Ar] 3d¹⁰. The d-orbitals are completely full and so no electron transitions between d-orbitals when the Zn²⁺ ion is in a complex. For this reason, zinc is often not described as a transition metal. 

The activity above is taken from Advanced Starters for Ten: Transition metal chemistry, which you might find a useful resource with your students.