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Element identification from emission spectra

Learn how to use the light analysis feature by performing an emission spectra experiment.

Scientists use models to make predictions, solve problems, and test claims. As technology improves, new evidence either confirms or contradicts previous models. Today’s atomic model is a product of centuries of refinement. In the early 1900s, Niels Bohr proposed an atomic model based on the work of many scientists, including Ernest Rutherford and Max Planck. Bohr was familiar with hydrogen’s line emission spectrum and realized the energy of emitted light must be related to an atomic structure that included energy levels. Bohr hypothesized that emitted light was the result of an electron jumping from one energy level to another. He proposed that the energy of an emitted photon is equal to the energy difference between the ground state and excited state. The distinct pattern of spectral lines unique to each atom could be related to energy levels.

Bohr’s model was critical in the development of modern atomic theory because it provided the idea that electrons are organized according to energy levels outside of an atom’s nucleus. Unfortunately, the model failed to fully explain the emission spectra of elements beyond hydrogen. The value in studying Bohr’s model lies in how it conveys a complex idea in a simple representation.

A spectrometer is a versatile tool that allows the observer to study the line emission spectrum of atoms with energized electrons. Spectrometers allow scientists to identify elements present billions of light years away from Earth, as well as provide evidence for the quantum model of the atom.

Gather materials

Safety

Follow these important safety precautions in addition to your regular classroom procedures:

  • Wear gloves when handling the spectrum tube.
  • Do NOT touch the high-voltage spectrum tube or power supply while they are active.
  • Use caution around the high-voltage spectrum tube power supply at all times.

Set up the equipment

  1. Connect the Spectrometer to Spectrometry.
  2. Place the rectangular end of the fiber optic cable in the cuvette opening of the spectrometer. Align the arrows on the side of the housing so that they point in the directly of the arrow in the illustration above.
  3. Use a multi clamp to secure the probe or rounded end of the Fiber Optic Cable on a ring stand, as shown above.
  4. Place the end of the probe a distance of 2 cm or less from the gas tube. Adjust the probe to point towards the tube. Do not allow the probe to directly touch the tube.

Collect data

  1. Select the Analyze Light page in the menu at the top of the page.
  2. Turn on the power supply.
  3. Click Record . Adjust the probe angle and distance from the gas spectrum tube if the wavelength reading is either too intense or too weak. You can also use the sliders on the left of the screen to adjust the integration time, number of scans to average, or smoothing.
  4. Click Stop when you have a stable reading, then turn off the power supply.

Analyze data

  1. Click Scale To Fit to scale the data.

  2. Use the Coordinates tool to find the wavelengths (in nm) of each distinct peak. Record each wavelength in a table like the one below.

    Color of Photon Wavelength (nm) Energy (J)
       
       
       
       
       

  3. Use the arrows at the bottom-right of the screen to view each reference spectrum. Which element is contained in the gas tube?

  4. Use the wavelength of the identified peaks to solve for the energy of emitted photons using the following equation:

    Photon Energy Equation

Questions to consider

  1. Which color has higher energy: red or blue?
  2. Is wavelength directly or indirectly related to energy?
  3. Is frequency directly or indirectly related to energy?

  4. Niels Bohr used mathematical relationships to predict the structure of the atom. He assumed lower energy electrons were closest to the nucleus and higher energy electrons were farther from the nucleus.

    1. Suppose an electron in its ground state (n = 1) were to absorb energy and transition to an excited state (n > 1), return to ground, and release a photon as it returns to n = 1. Which path will require less energy to complete, A or B?
    2. Which light color is more likely to result from the path you identified as less energy: red or blue?

  5. Astronomers use high-powered spectrometers to analyze light throughout space. How it is possible for an element to have the same line emission pattern every time it is energized, whether the element is in outer space or in a gas spectrum tube in a classroom?