Name: ______Date:______Period ______

Lab # 10 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:

Procedure: Follow website

Data:

Part 1: Flame tests and identification of an unknown metal.

Metal ion / Symbol of Ion with Charge / Observed Flame color
barium
calcium
sodium
rubidium
potassium
lithium

Part 1: Flame tests for unknown elements

Unknowns / Flame color / Identity of metal ion based on flame test
Unknown 1
Unknown 2


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 / Emission / Emission spectrum
Sodium
Neon
Mercury
Helium

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?