Atomic Spectra
Introduction:
Light and all other electromagnetic radiation is energy that is
emitted in the form of waves. Thus light behaves like a wave, and the
energy of light varies with the wavelength (the length of the wave) and
frequency of the light (number of waves that pass a point in a second).
The shorter the wavelength of light (the higher the frequency) the higher
the energy. Gamma rays and x-rays are high-energy radiation, radio
waves are low-energy radiation, and visible light is intermediate in energy
(wavelength falls between 350 and 650 nm). Matter can either absorb or
emit light, but when it does this, the matter gains or loses the exact
amount of energy that corresponds to the energy of that light. This
emission of light by matter is the basis for several common light sources,
including neon lights and sodium-vapor lights.
Atoms normally exist in a lowest energy, most stable state. If an
atom gains energy (absorption of light is one way to do this), it becomes
less stable and emits light to return to the more stable state. The atoms
of each element emit very specific light. In fact, the emission of light by
the elements is so distinctive that light is often used to determine which
elements are present in a sample. For example, when hydrogen gas is
zapped by an electric current, a pink color is given off. If this light is
passed through a prism, it is separated into four colors: violet, blue,
green, and red. The spectrum is referred to as discrete since only certain
colors are seen and the spectrum is not continuous.
In 1913, Niels Bohr explained the discrete spectrum of hydrogen
by relating it to the electron. Normally the electron in the hydrogen atom
is located in the first energy-level (shell). When a hydrogen atom atoms
gains energy, the electron moves from a lower energy-level to one of
higher energy. The energy gained by the atom is exactly the amount of
energy needed to move the electron from the lower energy-level to the
higher energy-level. With its electron in a higher energy-level, the atom is
now in an unstable, higher energy, excited state. The tendency is for
electrons to occupy the lowest level available. So shortly after gaining the
energy, the electron returns to a lower energy-level. Energy must be given
up when this occurs, and the energy is lost as light. Each line in the
emitted light of hydrogen represents the movement of an electron from a
specific outer (higher energy) level to a specific inner (lower energy) one.
The red glow of a neon sign is caused by neon atoms excited by
electricity. The many colors in fireworks are the result of excited metal
atoms.
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n=
n= 6
n= 5
n=4
infrared
n=3
visible
n=2
ultraviolet
Electron transitions in the hydrogen atom
n=1
This lab will allow you to view the emissions from spectrum tubes
containing elements in the gaseous phase. You will also test various salts
in a hot Bunsen burner to learn the expected color given off by an excited
atom. Both the spectrum tubes and flames will be viewed through a
spectroscope to allow you to see the visible spectrum generated by the
electrons as they give off energy and move between energy levels in the
atoms.
Procedure:
Solid metal salts in Bursen burner.
The solid salts are on the counter in small snap containers. Each
salt has a large card near it with its name on it. Just behind the salt is a
glass container or bottle containing several nichrome wires with labels
for that particular salt. Use the appropriate wire with its own salt.
Place a drop of distilled water on the loop of the wire and scoop up
some solid to be tested. Hold the solid in the hottest part of the flame.
Observe the color produced and record it on your report form. Next, while
one person burns the salt, the other should try to observe the flame
using rainbow glasses. Report any lines observed on the report form.
Repeat the above steps for all the solids available on the lab cart.
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Gaseous elements in spectrum tubes.
Gas spectrum tubes are placed throughout the lab area. Make sure
you turn off the power supply when you have finished with each tube.
Observe the color produced when each gas is first turned on and record
it on your report form. Next, observe the bright line spectrum for each of
the gases using your rainbow glasses. It will be necessary to lower the
room lights to see the lines from the spectrum tubes clearly. Record the
bright-line spectrum for each gas on your report form.
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Name
Date
Report for Atomic Spectra Experiment:
Data:
Ions in salt samples:
Compound
Color
Line Spectrum
LiCl
SrCl2
BaCl2
CuCl2
NaCl
KCl
CaCl2
Gases in spectrum tubes:
Gas
Color
Line Spectrum
Questions:
1.
A July 4th fireworks display is red when ignited. What element(s)
is/are probably present in the fireworks?
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2.
Suppose you are cooking spaghetti in water you have salted with
NaCl. You notice that when the water boils over, it causes the
flame of the gas burner to turn bright orange. How do you explain
the appearance of the colored flame?