Excited Elements

Introduction:

As electrons absorb energy they become excited and move to higher energy levels. As the electrons fall back to lower energy levels they release the energy they absorbed in set amounts called quanta. The energy that is released as electrons fall from higher to lower energy levels has a characteristic wavelength and frequency that corresponds to a particular type of electromagnetic radiation. For example, when electrons fall from a higher energy level down to the 2nd energy level, the wavelength and frequency of the energy produced correspond to that of visible light.

Electrons of atoms can be excited in various ways including heat, electricity and friction. For example, a solution of sodium chloride placed on a platinum wire and held in a flame emits a bright, yellow light that is characteristic of the metallic element sodium. Another method of spectrum analysis involves the application of high voltage across a gas-filled glass tube. Gases under low pressure and excited by an electrical discharge give off light in characteristic wavelengths. The emitted light is passed through a spectroscope, which breaks light into its components for analysis. A gas viewed through a spectroscope, such as the one shown in Figure 1, forms a series of bright lines known as a bright-line or emission spectrum. Since each element produces a unique bright-line spectrum or pattern, spectroscopy is a valuable branch of science for determining what elements are present. The composition of stars and other objects in outer space is determined using this technique. Unlike gases, heated solids produce a continuous spectrum.

A gas is identified by comparing the wavelengths of its emission (bright line) spectrum to the spectrum produced by a known gas. In this experiment, you will use a spectroscope to determine the bright-line spectra characteristic of different elements.

Purpose:

To observe the characteristic bright line spectra produced by applying high voltage across a sample of a gas at very low pressure and to determine the identity of an unknown gas.

Materials/Equipment:

High voltage power supplies / Spectral tubes:
Diffraction grating glasses / Helium
Spectroscopes / Neon
40-watt incandescent bulb/socket / Oxygen
Thermal mitt / Mercury
Fluorescent bulb/socket / Nitrogen
Colored Pencils (ROYGBIV) / Hydrogen
Label Cards for each spectral tube / Unknown

Safety Considerations:

  • DO NOT TOUCH the spectrum-tube power supply or spectrum tubes when power is applied. Several thousand volts exist at the power supply and spectrum tubes.

Procedure:

  1. Obtain a spectroscope and look through it at an incandescent light bulb. The spectrum should appear when the slit in the spectroscope is pointed just off center of the glowing filament. Practice moving the spectroscope until you see a bright, clear image.
  2. Darken the room but leave enough background lighting to illuminate the spectroscope scales. Point the spectroscope away from any exposed window, since daylight will affect the observed gas spectrum.
  3. Helium or hydrogen is a good first choice among the spectral tubes set up around the room. Adjust the spectroscope until the brightest image is oriented on your scale. Record in Table 1 the five brightest lines of the observed spectrum. Some of the spectrum tubes produce light so dim that you must be very close to them to get good observations of the spectral lines.
  4. Repeat for each of the other spectrum tubes.

Figure 1: Look through the spectroscope at the emitted light of the spectrum tubes.

Name ______

Partner’s Name(s) ______

Period ______Date______

Data:

Draw in lines at the proper locations on the scale and of the correct width and intensity to reflect what you have observed. On the line at the right of each scale, write the name of the element you are observingand the color given off by the bulb as seen without using the spectroscope.

4 / 5 / 6 / 7 / Name of Sample and Color of light
4 / 5 / 6 / 7 / Name of Sample and Color of light
4 / 5 / 6 / 7 / Name of Sample and Color of light
4 / 5 / 6 / 7 / Name of Sample and Color of light
4 / 5 / 6 / 7 / Name of Sample and Color of light
4 / 5 / 6 / 7 / Name of Sample and Color of light
4 / 5 / 6 / 7 / Name of Sample and Color of light
4 / 5 / 6 / 7 / Name of Sample and Color of light
4 / 5 / 6 / 7 / Name of Sample and Color of light
4 / 5 / 6 / 7 / Name of Sample and Color of light
4 / 5 / 6 / 7 / Name of Sample and Color of light
4 / 5 / 6 / 7 / Name of Sample and Color of light
4 / 5 / 6 / 7 / Unknown Color of light

Name ______

Partner’s Name(s) ______

Period ______Date______

Questions:

  1. Which type of spectrum is produced by an incandescent bulb?
  1. Observe a fluorescent light bulb with the spectroscope. While a continuous spectrum will be visible, you will also see a bright-line spectrum. Compare this spectrum with those of the gases observed in this activity. Which is most similar to the bright line spectrum from the fluorescent bulb?
  1. Compare the color of light emitted by the spectrum tube to that observed through the spectroscope. Can you predict the spectral pattern by looking at the tube with just your naked eye?
  1. Are all lines of the spectral patterns of the same intensity?
  1. Are the spectral lines always in the same order: ROYGBIV ?
  1. How do electrons produce emission spectra?
  1. Why are the lines different colors?
  1. Compare the spectral pattern of the unknown to the others observed. What is the identity of your unknown?

Applications

  1. Describe the uses of a spectroscope in the science of astronomy.
  1. How can spectra be used in chemical analysis?

ASIM Spectrophotometry: Excited Elementsp. 1

Revised: 10/09