CP
Chemistry
Laboratory
4
Light, Energy, and Electron Structure
Introduction
Sunlight passing through a prism produces a rainbow of colors – the visible spectrum. The
separation of white light into its component colors occurs when light waves of different
wavelengths are bent by different amounts. When a pure atomic gas such as hydrogen or helium is
subjected to a high – voltage electrical discharge, light is produced and the gas glows. When this
light is passed through a diffraction grating, however, the spectrum it produces is different. Instead
of giving the familiar rainbow of colors, the light emitted by the gas gives a series of bright, colored
lines. The series of bright lines is called an atomic emission spectrum and is unique to each element.
The relationship between the energy of light and its wavelength is
 E = hc / λ
Equation 1
E = _________, h = Plank’s constant = 6.626 *10-34 Js, c = speed of light = _________, λ = _____________
In this experiment you will be using an instrument called a spectroscope to view the line emission
spectra for several sources of light. The spectroscope contains a diffraction grating that separates the
light into its component wavelengths.
Observations will be made of various sources of light with the spectroscope. The emission spectra
of elements will be observed from spectrum tubes which contain element in the form of a gas.
These gas samples will be exposed to high voltage which will cause the sample to emit the light.
This is the same method used in fluorescent lights, street lights, and neon lights.
Objective
To recognize continuous versus line emission spectra for various sources of light using a
spectroscope. To view and record the line emission spectra for several different sources of light.
1
Procedure- You will work with a partner
1. Using the spectroscope observe the continuous “rainbow” spectrum from the incandescent
lightbulb.
2. Observe the colors of light in the visible spectrum and the wavelength range for each color
band. Sketch the spectrum of white light using colored pencils in the appropriate wavelength
boxes in the Spectrum Table. Note that the units of wavelength on the spectroscope are
nanometers. (1 nm = 10-9 m)
3. Observe the atomic emission spectrum of air, hydrogen, fluorescent, and two other tubes. In
the Data Table make note of the type of spectrum (continuous or line), and the number of
lines you observe (only note the bright lines), under the colors column indicate the color and
the wavelength of each color.
4. Using the colored pencils sketch the atomic spectrum of the incandescent, hydrogen, and
your two other tubes spectra in the wavelength boxes in the Spectrum Table.
5. Answer the post lab questions.
2
Atomic Spectra Report Sheet
Data Table
Light Source
Spectrum
Type
Colors, Wavelength (nm)
(Number of
Lines)
Fluorescent Light
Hydrogen Tube
Helium
Mercury
EXAMPLE:
Line
Bromine
(5 lines)
Violet line = 420 nm
Violet line = 425 nm
Violet = 450
Blue line = 475 nm
Blue line = 480 nm
3
Spectrum Table
Light
Source
700-650
650-600
600-550
550-500
500-450
450-400
nm
nm
nm
nm
nm
nm
Fluorescent
Light
Hydrogen
Tube
Helium Tube
Mercury Tube
Post Lab Calculations and Analysis
1. According to Equation 1 the energy of light emitted (E) is inversely proportional to its wavelength (λ) –
as the wavelength increases, its energy decreases (think low, low, long). Based on the spectrum observed
for fluorescent white light, rank the colors in the visible spectrum from highest energy to lowest energy.
2. Do all of the colors of light in the visible spectrum span about the same wavelength “width” i.e. do the
bands of color appear equally wide or narrow? If not, explain any differences.
3. What color of light in the visible spectrum appears brightest? Does this mean it is the highest energy
light? Explain your answer.
4
4. Using Equation 1, calculate the energy (E) corresponding to each line in the atomic emission spectrum
of hydrogen. Show your work.
5.
∆ E (J)
Color of Line
Transition
As shown in the figure below, the visible emission spectrum of hydrogen is due to transitions from
excited energy levels down to the second principal energy level (n = 2). Thus, the highest
energy violet line is due to the transition from n= 6 to n=2, and the lowest energy red line is due
to the transition from n=3 to n=2. Enter the energy values (Question 4) from the highest to
lowest in the following table and fill in the missing entries.
E5
E4
E3
E2
Violet Indigo
Blue
Red
5
Energy Difference
ninitial → nfinal
Violet
6→2
Red
3→ 2
6