Quick answer: Electrons fill subshells in order of energy, giving each atom a configuration. This AP Chemistry lab on photoelectron spectroscopy gives the aim, the equipment, the safety points, a step-by-step method, sample results and 7 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
When metal ions are heated in a flame, their electrons absorb energy and release it as light. In this lab you observe flame colours and use them to estimate photon energies, linking colour to energy levels.
Level: AP Chemistry · Curriculum: AP Chem Unit 1
Aim
To compare the colour of light emitted by metal ions in a flame, and relate the colours to electron energy levels.
Equipment
- Bunsen burner and heat-proof mat
- Wooden splints soaked in 0.5 mol/L solutions of lithium, sodium, potassium, calcium and barium chlorides
- Handheld spectroscope or diffraction grating
- Beaker of distilled water
Safety
Wear goggles and tie back long hair. Use only the solutions provided and keep them away from the flame. Put used splints into the beaker of water.
Method
- Light the burner and adjust it to a blue, non-luminous flame.
- Hold a soaked splint in the edge of the flame for 5 seconds.
- Record the colour, and view it through the spectroscope to note the strongest wavelength.
- Rinse the splint holder, wait for the flame to return to blue, then test the next solution.
Results
| Ion | Colour | Strongest wavelength (nm) |
|---|---|---|
| Li⁺ | crimson | 671 |
| Na⁺ | yellow | 589 |
| K⁺ | lilac | 766 |
| Ca²⁺ | orange-red | 622 |
| Ba²⁺ | green | 524 |
Analysis questions
Question 1 Photon energy in eV
Use E = 1240/λ (λ in nm, E in eV) to calculate the photon energy for each ion.Question 2 Photon energy in joules
Use E = hc/λ with h = 6.626 × 10⁻³⁴ J·s and c = 2.998 × 10⁸ m/s to confirm the energy for sodium, in joules.Question 3 Order of energies
Arrange the ions in order of increasing photon energy.Question 4 Origin of the light
Explain how an excited electron produces light of one specific wavelength.Question 5 Trend in group 1
Explain why lithium, sodium and potassium emit different colours although each has one valence electron.Question 6 Reliability
The colour of Na⁺ appears in every flame because of contamination. Explain how this could mislead the experiment and suggest a precaution.Question 7 Improvement
Suggest a method to measure the wavelengths more accurately than by eye.
