Analysis
of Spectral Lines
Why?
Many
devices give off visible light when heated or when an electric current is
passed through them. Everyday samples
include light bulbs, stove and toaster oven elements, and neon signs. Visible light may be separated into
individual colors of light by passing the light through a prism or a
diffraction grating (which is found in a spectroscope like the ones you used in
class!). White light and sunlight
consists of all the colors of the rainbow (ROY G BIV), but the light given off
by an excited element consists of light of specific colors only. A display, like those you say in the lab, of
the different colors is called an emission spectrum of the element. Just like fingerprints can be used to
identify an individual, an emission spectrum can be used to identify the
presence of an element.
Success Criteria
- Describe
how a bright line spectrum is produced in terms of energy transitions of
the electrons in an atom as they move from one energy level to another
- Qualitatively
relate the energy of electronic transitions to the specific colors of
light observed.
- Accurately
identify the presence of an element in an unknown mixture by comparing the
emission spectrum of a mixture with the emission spectrum of specific
element.
Resources
- Light
emission information from the lab for specific elements
- Modern
Chemistry, pp.
Prerequisites
- Bohr
model of the atom, visible light spectrum colors
Vocabulary
- In
the space below, define ground state, excited state, spectral lines, line
emission spectrum
Model 1: Production of an Emission Spectrum
In
Bohr’s atomic model an atom’s electrons are assigned to specific energy
levels. The atom is in its ground state
when the electrons occupy the lowest possible energy levels. When an electron absorbs sufficient energy
it moves to a higher energy level to produce an excited state. When the electron releases the energy, it
drops back to a lower energy level. The
energy is released in the form of light!
The wavelength of the light indicates the difference between the energy
of these two levels. Each wavelength of
light corresponds to a specific color of light (which may or may not be in the
visible area of the spectrum).
Consequently, atoms emit a characteristic set of discrete wavelengths,
not a continuous spectrum.
Since each element has
its own unique electron arrangement, the light that is emitted by the atoms
produces an emission spectrum that can be used to identify the element. In other words, an atomic spectrum can be used
as a “fingerprint” for an element because it is unique for each element and
reflects the energy levels occupied by the electrons in an atom of the element.
Task:
First, determine what color is observed for each
of the gas samples below.
|
Metallic
Element |
Color
of the Flame |
|
Barium |
|
|
Calcium |
|
|
Lithium |
|
|
Potassium |
|
|
Strontium |
|
|
Sodium |
|
Key Questions
1. What is the difference between an excited and
ground state of an atom?
2. What happens when electrons move from the
excited state to the ground state?
3. What colors of visible light are found in the
emission spectrum of hydrogen?
Model 2: Energy Level Diagram for the Hydrogen Atom
If atoms only emit discrete wavelengths of light,
then an atom’s energy levls can only have discrete energies. The energy level diagram below illustrates
some of the energy levels found in a hydrogen atom, with arrows showing the
corresponding electron transitions that produce its visible emission
spectrum. The transitions shown are
from excited states to the second energy level. Transitions to ground state do not fall in the visible range.
violet
blue green red
Key Questions
4. What happens to the energy of the electrons to
produce the observed spectral lines?
5. For the element hydrogen, which color
(wavelength) of light is produced by the largest energy drop of an
electron? Explain your answer (based on
the model).
6. Which color of light is produced by the
smallest energy drop of an electron?
Explain your answer.
7. Why do different elements emit different colors
of light?
EXTRA CREDIT!
8. In Model 2, all of the transitions stop at
n=2. If a transition were to be shown
in the diagram, would you expect the light to be in the infrared or ultraviolet
region of the spectrum? Explain your
answer.
Problems:
1. Below are the bright line spectra of four
elements and the spectrum of an unknown gas.
a) Which
elements are in the unknown?
2. In a forensic investigation, it was suspected
that the victim was poisoned by a toxic chemical which contains the element
copper and accumulates in large amounts in hair as well as other tissues. Using your knowledge of emission spectra,
design a method by which you can confirm the presence of this toxin in the
victim. Include the resources and
equipment you would need to make a definitive determination.