Download Lab Atomic Spectra

Name: _______________
Date:_________
Lab # 10
Period ________
Due Date:_______
Spectroscopy: Element Identification and Emission Spectra
Introduction: (This is a computer lab)
The energy levels in atoms and ions are the key to the production and detection of light. Energy levels or
"shells"exist for electrons in atoms and molecules. The colors of dyes and other compounds results from electron
jumps between these shells or levels. The colors of fireworks result from jumps of electrons from one shell to
another. Observations of light emitted by the elements are also evidence for the existence of shells, subshsells
and energy levels. The kinds of light that interact with atoms indicate the energy differences between shells and
energy levels in the quantum theory model of the atom. Typically the valence electrons are the ones involved in
these jumps.
Atoms have two kinds of states; a ground state and an excited state. The ground state is the state in which the
electrons in the atom are in their lowest energy levels possible (atoms naturally are in the ground state). This
means the electrons have the lowest possible values for "n" the principal quantum number.
Specific quantized amounts of energy are needed to excite an electron in an atom and produce an excited state.
The animation shows the opposite of excitation. It shows how the excited hydrogen atom with an electron in the n
= 3 shell can release energy. If the electron in hydrogen only drops to the n = 2 shell the energy matches a pulse
of red light.
Note the size of the electron cloud in the excited atom changes when the electron moves from shell to shell.
The size of the atom decreases in volume when the electron goes from the n=3 shell to the n = 2 shell. On
average the electrons are closer to the nucleus for lower values of "n". The electron cloud is related to the most
probable distance between the nucleus and the electron. The most probable distance increases with increasing
"n" value. The excited electron is still "in" the atom even in an excited state. The valence electron will only escape
the atom if the electron is given an amount of energy equal to the ionization energy for that atom.
Materials:
Online website: http://www.800mainstreet.com/spect/emission-flame-exp.html#Anchor-tubes
Procedure: Follow website
Data:
Part 1: Flame tests and identification of an unknown metal.
Metal ion
barium
calcium
sodium
rubidium
potassium
lithium
Symbol of Ion with Charge
Observed Flame color
Part 1: Flame tests for unknown elements
Unknowns
Unknown 1
Unknown 2
Flame color
Identity of metal ion
based on flame test
Part 2 Observing line spectra with the spectroscope
In the second part of the experiment you will observe the color of light emitted by excited gases of elements in
sealed glass tubes called "spectrum" tubes. Direct current, DC, high voltage electrons are used to excite the
atoms in the spectrum tube. High voltage means 1000 to 2000 volts. This is more than 10 times normal
household voltage which is 120 volts AC.
The excited atoms release the energy they gained. Some of this energy is in the form of heat and some is in the
form of light. The billions of excited atoms release energy. Each excited atom releases a single pulse of light
energy as it returns to the "ground" state or low energy state. There are so many pulses emitted the light appears
to be continous.
The excited atoms do not all emit the same energy light because the amount of energy that excited them may
differ, but there are limitations on the colors they do emit. The kind of light depends on the size of the gaps
between the "shells" or energy levels in the atom. The electrons are changing "n" values in the atom. Remember
"n" can have only positive whole number values like 1, 2, 3, ... up to infinity.
The kind of light energy that can be emitted by excited atoms is unique for an element. The pattern of "lines' or
colors emitted can be used to identify an element. An powerful extension of this is the ability to measure amounts
of an element by measuring the brightness of the emitted light.
A spectroscope can separate the light produced by an emission tube. The color seen by the naked eye is a
combination of a number of colors of light. These are separated by a prism or a diffraction grating which acts like
a prism. The emission lines can be seen when you look through the spectroscope at the light source. You will be
able to observe the "line" spectrum for the elements and record the spectral line
Part 2 Emission line spectra for selected elements
Element
Sodium
Neon
Mercury
Helium
Emission
Emission spectrum
Analysis Questions: Directions: Answer all questions and be sure to write a valid conclusion for this
lab.
How do these emission spectra compare in terms of colors and numbers of emission
line positions?
Are the spectra identical?
What if anything is similar?
What is different?
Examine the spectra for the elements Na, Ne, Hg or He and answer the following
questions.
FILL IN THE FOLLOWING TABLE WITH YOUR ANSWERS
Element with greatest number of visible emission __________________
lines
Longest wavelength in the spectrum of this atom in __________________
nanometers.
__________________
Color of light for this longest wavelength
Examine the spectra for the elements Na, Ne, Hg or He and answer the following
questions.
FILL IN THE FOLLOWING TABLE WITH YOUR ANSWERS
Element with fewest number of emission lines
__________________
Longest wavelength in the spectrum of this atom
__________________
in nanometers
__________________
Color of light for this longest wavelength
What suggestions do you have additions or changes to this experiment?
What "new" idea did you learn from this experiment?
Why does a sodium vapor street light look yellow instead of white?
What would you expect to happen to the size (volume) of a hydrogen atom when the
outer electron moves from the n = 2 shell to the shell with n = 4 ?
Would the volume increase?