AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
D electrons move from the ground state to an excited state when light is…
Full specification wording
Formation of coloured ions (A-level only): d electrons move from the ground state to an excited state when light is absorbed.
In a free gaseous ion the five d orbitals have the same energy.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 3 of 6
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
In a free gaseous ion the five d orbitals have the same energy. Ligands approaching the metal repel electrons in the d orbitals pointing towards them more than those pointing between them, so the set splits into a lower group and an upper group separated by an energy gap ΔE. An electron in the lower group can absorb a photon whose energy is exactly ΔE and jump to the upper group; that is the ground state to excited state transition, and the missing photons produce the colour. Two conditions must hold: there must be electrons in the lower set to promote, and spaces in the upper set to promote them into. Sc³⁺ is 3d⁰ and Zn²⁺ is 3d¹⁰, so neither can do it and both are colourless.
What earns the marks
- Stating that the d orbitals are split into two energy levels by the ligands.
- An electron being promoted from the ground state to the excited state by absorbing light.
- The energy of the absorbed photon being equal to ΔE.
- Explaining a colourless ion such as Sc³⁺ or Zn²⁺ by its empty or full d sub-level.
Common mistakes and how to put them right
Saying the electron is promoted from the 3d to the 4s sub-level rather than between the split d orbitals.
Describing the electron as being emitted from the ion rather than promoted within it.
Claiming the absorbed energy can take any value rather than matching the gap exactly.
Saying Zn²⁺ is colourless because it has no d electrons, when it has a full 3d¹⁰ sub-level.
Exam tips
- Use the words ground state and excited state, because the mark scheme uses them.
- Draw the split as two horizontal levels with the gap labelled ΔE whenever a diagram is allowed.
- Check the d electron count before claiming that a species is coloured.
Try the lesson
The start of the Formation of coloured ions (A-level only): d electrons move from the ground state to an excited state when light is absorbed. lesson, made from checked teaching notes for this specification point.
Have a go first
When a transition metal ion absorbs visible light, what happens to a d electron?
Listen
In a free gaseous ion the five d orbitals have the same energy. Ligands approaching the metal repel electrons in the d orbitals pointing towards them more than those pointing between them, so the set splits into a lower group and an upper group separated by an energy gap ΔE. An electron in the lower group can absorb a photon whose energy is exactly ΔE and jump to the upper group; that is the ground state to excited state transition, and the missing photons produce the colour. Two conditions must hold: there must be electrons in the lower set to promote, and spaces in the upper set to promote them into. Sc³⁺ is 3d⁰ and Zn²⁺ is 3d¹⁰, so neither can do it and both are colourless. A strong answer earns marks for these points. stating that the d orbitals are split into two energy levels by the ligands. an electron being promoted from the ground state to the excited state by absorbing light. the energy of the absorbed photon being equal to ΔE. explaining a colourless ion such as Sc³⁺ or Zn²⁺ by its empty or full d sub-level. Exam tip: Use the words ground state and excited state, because the mark scheme uses them.
Key words
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ground state and excited state
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photon energy matches the gap
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See every definition
- ground state and excited state
- An electron in the lower group absorbs a photon and jumps to the upper group, which is the ground state to excited state transition.
- photon energy matches the gap
- An electron in the lower group can absorb a photon whose energy is exactly ΔE and jump to the upper group.
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