Emission Spectroscopy or Atomic Spectroscopy Chemistry Tutorial

Key Concepts

• An emission (atomic) spectrum is produced when a gas is heated.
• The atoms in the gas absorb energy, causing some electrons to move from the lower energy ground state to a higher energy excited state.
  

  excited state	--------	higher energy
  electron absorbs energy	↑ | | | | |
  ground state	--------	lower energy

• These excited electrons will fall back to the lower energy ground state and emit the energy they had previously absorbed.
  

  excited state	--------	higher energy
  electron emits (releases) energy	| | | | | ↓
  ground state	--------	lower energy

• The energy released corresponds to particular wavelengths (and frequencies) of light.
  
  The greater the energy released, the shorter the wavelength of light emitted.¹
• This emitted light can be viewed through a spectroscope to produce a line emission spectrum, a series of coloured lines on a dark background.
• The line emission (atomic) spectrum of an element is unique.
• Line emission spectra (atomic spectra) can be used to identify the presence of an element in a sample because no two elements produce the same line emission (atomic) spectrum.

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Examples of Line Emission Spectra

lithium
sodium
potassium
rubidium
cesium
calcium
strontium
barium
zinc
arsenic
selenium
wavelength	400nm	450nm	500nm	550nm	600nm	650nm	700nm

Example of Element Identification

Identify the elements present.

1. Compare the line emission spectrum of the unknown salt to the spectra of the various elements shown above.
2. Notice that there is a red line between 600 nm and 650 nm which could correspond to the line emission spectrum of either lithium or calcium.

   lithium
   unknown
   calcium

   Most of the other wavelengths for calcium are missing from the spectrum of the unknown, so we disregard calcium and decide that lithium is one of the elements present in the sample.

3. Next we try to match up the yellow lines, one of which appears as a band (thick line).

   Looking at the spectra of the elements above, the most likely element is potassium, so we compare the spectrum of potassium and the known.
   

   lithium
   unknown
   potassium

   The spectral lines of potassium are a good match for the remaining spectral lines of the unknown.

4. The unknown salt sample contains lithium and potassium.

Exam Questions

• white lines on a black background
  

  potassium

• black lines on a white background²
  

  potassium