Excited Elements/Emission Spectra Teacher Notes



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Excited Elements/Emission Spectra

Teacher Notes


COS Objectives:

Standard 3:  Periodic Trends

Standard 3:  Bullet 3:  Historical Development of the Atomic Table

Lab Time: The lab can be completed in 30-45 minutes

Preparation:


Time: 15 minutes to set up the tubes
Station label cards have been included in the box. It is strongly suggested that you set up your stations to match the order of bulbs and tubes listed on the answer sheet for a smoother flow of students around the room.

There are two versions of the answer sheet. Copy only 1. Choose the one that matches the orientation of the scale on the particular type of spectroscope that you are using.

If you are using the diffraction grating glasses, remind students to record only one spectral pattern (usually the one in the right horizontal position) since several orientations are seen at once.

Considerations:


One of the difficulties of this lab is aligning the slit in the spectroscope with the narrow line of each of the discharge tubes. Students sometimes don’t realize what it is exactly they are looking for in the spectroscope so the observance of any light at all can quickly (an incorrectly) be assumed to be the line spectrum we are looking for.

To help the student understand what they are looking for you should probably explain what line spectra are prior to doing this lab. Another helpful trick is to allow them to see the continuous spectrum of an incandescent bulb and then the line spectrum that can be observed from a fluorescent bulb simply because it is a lot easier to align the spectroscope with these wider light sources. However, depending on the fluorescent coating on the fluorescent bulb, many typically show not only a line spectrum but also a semi-continuous spectrum overlapping each other. The line spectra which comes from the excited gas within the bulb (which emits some vision and a lot of UV-radiation) and the semi-continuous spectrum from the fluorescent coating on the outside glass which is excited by the UV-radiation emitted from the gas and then fluoresces a continuum in the visible)

During the experiment another helpful trick is to slowly move the spectroscope gently to the left and right as if your hands were shaky. In this way if you are having problems aligning the spectroscope with the narrow light source you are sure to pass by occasionally and the line spectra will become apparent.

Since we do the lab as multiple stations sometimes it appears that students who start off at the hydrogen bulb (with 4 lines) station seem to get what a line spectra is while students who start at the argon (with 60 lines) station claim they only see a rainbow and don’t understand what you are talking about.



Post Lab:

Once the lab is completed you should have the students note that for a given color in the visible spectrum they appear to always appear pretty much in the same location in the line spectrum. Any slight change in location is probably due to a slight change in the shade of color.

Answers to Questions:


  1. Which type of spectrum is produced by an incandescent bulb?

continuous


  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? (accept any reasonable answer – it used to be mercury, but has now changed)




  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? no




  1. Are all lines of the spectral patterns of the same intensity?

no


  1. Are they always in the same order: ROYGBIV ?

yes


  1. How do electrons produce emission spectra? When electrons are excited by heat or electricity, they move into higher energy levels. When they return to their “ground state”, the energy must be lost. In this case it is lost in the form of visible light.




  1. Why are the lines different colors?

Color is a function of wavelength and frequency and therefore, the amount of energy that is lost.


  1. Compare the spectral pattern of the unknown to the others observed. What is the identity of your unknown? ( we are currently using hydrogen)



Applications


  1. Describe the uses of a spectroscope in the science of astronomy.

Astronomers use spectra to determine the composition of stars.


  1. How can spectra be used in chemical analyses?

As a means of identifying an unknown, since each element produces a unique spectrum due to its unique electron arrangement.

ASIM Excited Elements Teacher Notes p.

Revised: 5/06




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