Quick answer: An atomic orbital is a region around a nucleus that holds at most two electrons, and orbitals come as s, p and d types. This Chemistry 11-12 lab on ionisation energy 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.
Why do salts of different metals colour a flame differently? In this lab you view flame colours through a hand spectroscope, read the wavelengths and convert them to photon energies, linking electron energy levels to the ionisation energies you studied.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: A Level 3.1.1, IB S1.3
Aim
To observe the colours that metal ions give in a flame, to read the wavelength of each colour with a hand spectroscope, and to calculate the energy of the photons emitted.
Equipment
- Bunsen burner and heatproof mat
- Nichrome wire loop or wooden splints soaked overnight in distilled water
- Solutions of the chlorides of lithium, sodium, potassium, calcium and barium (0.5 mol dm⁻³)
- Dilute hydrochloric acid (2 mol dm⁻³) in a small beaker
- Hand spectroscope with a wavelength scale in nm
- Safety goggles
Safety
Wear goggles. Tie back long hair. Barium salts are toxic, so wash hands after use and do not taste anything. Keep the dilute acid away from the eyes and never point the spectroscope at the Sun.
Method
- Light the Bunsen burner and adjust it to a blue flame.
- Clean the wire loop in the acid, then heat it until it gives no colour.
- Dip the loop in the first solution and hold it at the edge of the flame.
- Note the flame colour, then look at the flame through the spectroscope and read the wavelength of the brightest line.
- Clean the loop again and repeat for each solution.
Results
A student in Lagos recorded these readings (uncertainty ±5 nm).
| Ion | Flame colour | Wavelength / nm |
|---|---|---|
| Lithium | crimson | 670 |
| Sodium | yellow-orange | 589 |
| Potassium | lilac | 767 |
| Calcium | orange-red | 622 |
| Barium | pale green | 554 |
Analysis questions
Question 1 Photon energy of sodium
Use E = hc/λ with h = 6.626 × 10⁻³⁴ J s and c = 3.00 × 10⁸ m/s. Calculate the energy of one photon of the sodium line, in joules.Question 2 Energy per mole
Convert the sodium photon energy to kJ per mole using 6.022 × 10²³ mol⁻¹.Question 3 Completing the calculation
Calculate the energy per mole, in kJ mol⁻¹, for lithium and potassium.Question 4 Origin of the colours
Explain how a metal ion in a flame produces light of one colour.Question 5 Comparison with ionisation
The first ionisation energy of sodium is 496 kJ mol⁻¹. Compare it with your sodium value and explain the difference.Question 6 Reliability
Calculate the percentage uncertainty in the sodium wavelength and the uncertainty in its energy per mole.Question 7 Improving the method
Propose two changes to the spectroscope work that would tighten the wavelength readings.
