SPECTROSCOPY
Purpose
The purpose
of this activity is to observe the characteristic colors of metal ions using
flame spectroscopy and the patterns of atomic line spectra from a gas
discharge tube.
Introduction
Atoms and molecules
are able to absorb energy by a number of different methods and later emit the
energy in the form of light. An
incandescent light bulb converts electrical energy into heat, and the hot wire
re-radiates the energy as light. An
ideal black body is able to absorb and emit all colors of light equally well,
but no object is a perfect black body.
Therefore, heated materials usually have a particular shading or color
associated with them. Tungsten is chosen
as a filament in bulbs because it shows very little of this effect.
Another way
to produce radiant energy is to heat a material in a flame. The energy and
temperature of a flame is so large than no chemical bond can survive those
conditions. Therefore, the color of a
flame must be caused by individual atoms, not molecules. Different atoms show certain color preferences
based on their unique chemical natures.
Fluorescent
light bulbs are an example of a third way to produce light. Atoms in a fluorescent tube also absorb
electrical energy, but
the internal volume of a fluorescent bulb is a rarified
gas. Electrons are accelerated by high
voltage through the bulb and individual electrons collide with individual
atoms, transferring a large amount of energy to a gaseous atom as a
"packet". Atoms release this
energy spontaneously, and the "white" light of a flourescent
is actually the re-emission of the energy by an arsenic-containing powder
coating the inside of the glass tube.
Whenever an
atom releases energy by collisions with nearby neighbors, all colors of light
are produced; but when an atom absorbs and releases energy in isolation, only a
certain set of discrete frequencies are absorbed and released. Spectroscopy is the study of light
frequencies absorbed and released by atoms and molecules, which is the atom's
or molecule's characteristic spectrum.
In this
activity, qualitative observations will be made of atomic spectra using both
flame spectroscopy and discharge tubes.
In Part One, metal chlorides will be heated in a Bunsen burner flame. Testing hydrochloric acid in a flame, HCl, shows a colorless flame; therefore, the color of the
flame must be due to the metal ion alone.
In the same way, heating the metal scoop alone which is used to hold the
samples shows no color, either. In Part
Two, we will separate the individual colors emitted by the gas using a
diffraction grating, and observations of different elemental and molecular
gases clearly shows that each chemical substance has a characteristic pattern
of light frequencies.
It can be
shown that the flame spectrum of an atom, if the colors of light were separated
using a diffraction grating, would match the spectrum obtained from a gas
discharge tube.
Instructions
In Part
One, the instructor will place small amounts of different metal chlorides on
the tip of the metal scoop and heat it in the flame of a Bunsen burner. Record the color of each flame.
In Part
Two, spectrum boxes and diffraction gratings will be available for the viewing
atomic spectra. Sketch the spectra of
helium, mercury, neon, and hydrogen on the report sheet. Note both the relative positions of the lines
and their relative intensities. Attempt
to describe or characterize each spectrum briefly with words.
Report
Sheet
Part One:
Flame Spectra
The
instructor will place small amounts of different metal chlorides on the tip of
the metal scoop and heat it in the flame of a Bunsen burner. Record the color of each flame.
Compound Color
lithium chloride
sodium chloride
potassium
chloride
magnesium
chloride
calcium
chloride
barium
chloride
nickel
chloride
cobalt
chloride
copper
chloride
iron
chloride
Does there
appear to be any relationship between the color of the flame and the periodic
chart?
Part
Two: Line Spectra
Violet Blue Green
Yellow Orange Red
400 450
500 550 600
650
hydrogen
helium
neon
mercury
Does there appear to be a relationship between the lines of a given element, or between elements based on the periodic chart?