Emission Spectra

Category: 
Demonstration
Topics: 
Atomic Structure
Elements and Compounds
Quantum Theory

Discharge tubes containing samples of various elemental gases are placed in a high-voltage power supply. Students observe the emitted light through diffraction gratings. They see discrete line spectra corresponding to the energy level transitions of the electrons of the atoms of the element. 

Curriculum Notes 

This demo is usually performed when the electronic structure of the atom is being discussed. Students can pick up the gratings on their way into class and leave them on their way out. Allow about 15 minutes for this demo.

One day of lead time is required for this project.

Discussion 

Electrons traveling through the rarified gas in the tube collide with the gas atoms and excite their electrons into higher energy states. As the electrons return to ground state, they emit discrete frequencies of electromagnetic radiation, some of it in the visible range. Most instructors start with hydrogen and discuss its relationship to the Bohr model before proceeding to elements with more complex electronic structures.

Materials 

  • various discharge tubes (hydrogen, helium, nitrogen, neon, argon, and krypton)
  • high voltage power source for lamps
  • heat resistant glove
  • diffraction gratings (1 for each student)

Procedure 

  • Distribute diffraction gratings to the students. Instruct them to observe the spectra by holding it close to their eye and looking off to the side. Have them practice looking at the house lights to see if they can see a continuous spectrum (rainbow).
  • Make sure that the power supply is turned off. Attach one of the discharge tubes to the power supply.
  • Dim the house lights and turn on the power supply.
  • After observing and discussing the spectrum, turn off the power supply and remove the tube.
  • Repeat the process for each spectrum that you wish to observe.

Safety Precautions 

  • The discharge tubes get quite hot. Handle them with the glove provided to avoid getting burned.
  • The power supply is high-voltage. Make sure that the power is off when inserting or removing a tube.
  • If you pack or unpack the tubes from their cases, be sure to hold them sideways. Otherwise they sometimes fall out of the case.