EMISSION SPECTRA

DETERMINATION OF WAVELENGTH OF ATOMIC EMISSION LINES

Background

The most stable electron configuration for an atom is referred to as the "ground state".  In the ground state, all electrons exist in the lowest available energy level.  When atoms absorb energy from electricity or a flame, their electrons may absorb enough energy to jump to higher energy levels.  The atom is then said to exist in an "exited state".  The exited state is an unstable condition, and the electrons will tend to return to the ground state.  When exited electrons return to the ground state, they emit their excess energy as photons (packets of electromagnetic radiation).  If the emitted photons have wavelengths between ~400 to ~700 nanometers, they can be detected as visible light.  The collection of all colors of visible light of an element is called the element's emission spectrum.  Each spectrum consists of a series of bright lines of definite wavelength.  Each wavelength can be mathematically related to a definite quantity of energy produced by the movement of an electron from one discrete energy level to another.  Emission spectra are experimental proof that electrons exist in quantized energy levels within an atom.

 In this experiment you will determine the wavelengths of the bright lines of the emission spectra of hydrogen, helium and mercury.

 Procedure in brief:

A experimental setup will be placed in the lab allowing you to measure the wavelengths of the emission spectra of various species including hydrogen and mercury. You will be looking at the emission tubes through a grating which will separate the lines. The experimental apparatus will look something like:

You can determine the wavelength of the line of interest by the Bragg equation, l = d sin(q/2) where d is the distance between the individual lines in the grating.

Hydrogen (3 lines) Spectra: Knowing the wavelength of a particular line, you will need to determine which energy levels are involved the transition. You will need to use the equation for an electron bouncing between two energy levels in a hydrogen atom determine n_i. For all the transitions that you will observe for this lab, n_f = 2, as the transitions to n_f = 1 are beyond the visible range.  Remember, the value of delta E is negative (this is an EMISSION spectra) If you use the Rydberg equation with a positive E value, you will get a wrong number (like something between 0 and 2.4).

Mercury (3 lines), helium (6 lines) Spectra: Compare the values determined in the prelab to those found in the laboratory. Report % error for each line in each species.

12 min screencast desciption of the lab

Your 'report' will consist of two sections. For the hydrogen data, you need to give (in a nice computer generated table) values for d, sin (theta/2), wavelength, frequency, energy and n_i for each transition. For the other (non -H) emission spectra, you will report only sin (theta/2), wavelength and % error for each line. At least one sample of each and every calculation done in lab needs to be shown. Due to the ease of this experiment and needed report, it will be worth only 15 points.

Prelab questions: (To be written on separately on a piece of recycled paper)

Re-read teh section on emission spectra in your text.

1. What is a diffraction grating? What does it do? Where did you get this information?

2. Do some research and find the visible emission lines for mercury and helium. List them (actual wavelengths, not ranges or 'colors') and give your source. A google search of emission spectrum (plural: spectra) or absorption spectrum might be a good start. You are looking for the 3 most intense lines for mercury and 6 most intense lines for helium. It is these lines that your will be 'searching for' in lab and you will be calculating % error from them. This is meant to take you a bit, and you will learn about emission spectra as you search. This is not meant to be an 'easy' question. Do not 'guess' the values from some graph or picture, find a table of values.

~MEO (lots of thanks to JWS) 19 Sep 2007