Atomic Spectroscopy

Atomic Spectroscopy Lab

In these experiments we will study the electronic structure of atoms by inducing the atoms to absorb or emit various wavelengths of light. Using a hot gas flame we can cause atoms to emit wavelengths of light as the atom's electrons relax back to their ground states. Alternately we can study the excitation process by observing which wavelengths of light are absorbed as light passes through a solution containing the atoms being studied.

In the emission experiments we will excite the atoms of various elements by heating salt solutions containing these elements in a hot gas flame. Experience shows that the colour of the pale blue, gas flame often changes dramatically when salt solutions are heated. Sodium chloride causes the flame to burn bright yellow, whereas copper sulfate produces a bright green flame.

The electrons of an atom occupy orbitals. Orbitals describe the various ways in which an electron can exist around an atom. Each orbital has a discrete energy associated with it. Some orbitals have lower energies and others have higher energies. Under room conditions, the electrons of an atom fill the atom's lowest energy orbitals.

The diagram shows the energy levels for sodium. Sodium's ground state electronic configuration is 1s²2s²2p⁶3s¹

To move from one orbital to another an electron must absorb or emit energy which is exactly equal to the energy separation of the orbitals. When atoms are excited by the heat energy of the flame, electrons absorb energy and are promoted to empty higher energy orbitals. As these atoms 'relax', the electrons return to lower energy orbitals with the emission of energy in the form of light. If this light occurs within the visible region of the spectrum we see a coloured flame.

In an atom or molecule there are often many empty orbitals to which an electron can be promoted and we can see emission or absorbance of more than one wavelength of light. The human eye sees light containing multiple wavelengths as a single colour but by using a spectrometer we can resolve the component wavelengths.

Light can be separated into its component wavelengths by using prisms or diffraction gratings. Diffraction gratings consist of a series of accurately ruled grooves on a flat surface. When light of different colours hits the surface it is reflected (reflection grating) or transmitted (transmission grating) at different angles that depend on the light's wavelength. In our spectrometer we will use a transmission diffraction grating. Looking through the grating shows different colours of light at different angles.

When light passes through a narrow slit, the light emerges as a circular wave.

When light passes through a narrow slit, the light emerges as a circular wave.

If two slits are placed side by side, the emerging waves will interfere with each other; reinforcing in some locations and canceling in other locations. This is called constructive and destructive interference.

When light passes through a narrow slit, the light emerges as a circular wave.

If two slits are placed side by side, the emerging waves will interfere with each other; reinforcing in some locations and canceling in other locations. This is called constructive and destructive interference.

Changing the wavelength of the light will cause the regions of interference to shift. This is the basis by which a diffraction grating is able to resolve different wavelengths of light.

When a grating with many evenly spaced grooves is used the intensity of the transmitted light will be strong only when viewed from one angle. This angle θ depends on the light's wavelength λ and the groove spacing d.

Sin(θ) = λ/d

A light source viewed through a diffraction grating shows coloured copies of itself displaced horizontally. Colours with longer wavelengths are displaced more because their diffraction angle is greater. By measuring the displacement of the image a you can determine the wavelength of the light using the equation below. Make sure to record this equation as you will need it to complete the experiments.

Emission Spectroscopy

In this experiment you will study the emission properties of five metallic salts dissolved in water. You will heat each salt solution in a hot gas flame and record the flame colour and estimate its wavelength based on that colour. Next you will use a simple spectrometer to make measurements on the emission lines for each salt and estimate their wavelengths. Finally you will perform the same measurements on two unknowns. The first unknown is one of the five salts you examined. The second unknown is a mixture of two of the salts. By comparison to the emission spectra of the five metallic salts you will identify the species present in each unknown.

Unknown 1 sample number is

Unknown 2 sample number is

The spectrometer is controlled in the following fashion. The upper view shows the flame. The lower view shows the view through the diffraction grating. Pressing the flame button causes a platinum loop containing a drop of salt solution to be inserted into the flame.

The colour lasts for only a few seconds so will need to repeat this step several times until you have observed and recorded all the emission lines in your lab notebook. When you are done with a salt select the next salt solution by pressing its button. You can examine the salts in any order and for any length of time that you want. When you are completely satisfied that you are done, press the done button.

Before you can calculate the wavelengths of the emission lines you must know the value of d for your diffraction grating. You will determine this value by measuring the position of a strong sodium emission line, actually a pair of lines occurring at 589.592 and 588.995 nm. Since you cannot resolve this doublet with this equipment use 589.3 nm as the sodium wavelength and use this value to calculate d.

For sodium record the flame colour as well as the colour and position of the strong emission line in cm. Assume that the wavelength of this line is 589.3 nm and that the distance, l, between the diffraction grating and the slit is 40.0 cm. Use this information to determine the value of d.

For sodium record the flame colour and the colour and position in cm of the strong emission line. Assume that the wavelength of this line is 589.3 nm and that the distance, l, between the diffraction grating and the slit is 40 cm.

For lithium record the flame colour as well as the colour and position in cm of two or more emission lines. Calculate the wavelengths of the emission lines.

For cesium record the flame colour as well as the colour and position in cm of two or more emission lines. Calculate the wavelengths of the emission lines.

For calcium record the flame colour as well as the colour and position in cm of two or more emission lines. Calculate the wavelengths of the emission lines.

For strontium record the flame colour as well as the colour and position in cm of two or more emission lines. Calculate the wavelengths of the emission lines.

For this unknown sample record the flame colour as well as the colour and position in cm of two or more emission lines. Calculate the wavelengths of the emission lines and identify the unknown species.

For this unknown sample record the flame colour as well as the colour and position in cm of two or more emission lines. Calculate the wavelengths of the emission lines and identify the two species in the mixture.

Absorption Spectroscopy

In this experiment you will observe the spectrum of an electric fluorescent light source. This spectrum consists of various bands and lines that depend on the specific phosphors used in the lamp's manufacture. You will record the position, colour and intensity of the stronger bands and lines.

Then the spectrum of the same light source will be observed after the light has passed through various coloured solutions. In this experiment two concentrations of aqueous potassium permanganate will be used. In all cases you should pay attention to differences between the various spectra. You will record any changes in the intensity (brightness) and location of bands and lines. Three important lines or bands should be sufficient.

The spectrometer is controlled in the following fashion. The upper view shows the lamp and a solution in a vial. The lower view shows the view through the diffraction grating. Pressing the lamp icon turns on the lamp. Pressing it again turns off the lamp.

When the lamp is on, pressing the view icon will place the vial containing the solution in front of the lamp. Pressing it again will remove the vial. You can change solutions by pressing the appropriate vial icon. You can view the solutions in any order and for as long as you want. When you are completely done press the done icon.

The distance between the diffraction grating and the slit remains 40.0 cm as before. Use the value of d you calculated previously to relate the positions of lines and bands to their wavelengths.

Record the intensity, location and colour of the bands or lines produced by the fluorescent light. Six to eight bands should be sufficient.

Record changes in the intensity, location and colour of the bands or lines produced by the fluorescent light when filtered by the solution. Three major bands or lines should be sufficent.

The Effect of Complexation on a Copper Salt

In this experiment you will observe the spectrum of an electric fluorescent light source. Then the spectrum of the same light source will be observed after the light has passed through two solutions, one containing copper (II) sulfate and the other containing a complex formed during the reaction of copper sulphate (II) with ammonia.

In all cases you should pay attention to differences between the various spectra. You will record any changes in the intensity (brightness) and location of bands and lines. Three important lines or bands should be sufficient.

Copper (II) sulphate dissolved in water produces a light blue solution. Watch carefully as we added ammonia to the solution in a stepwise fashion. Record your observations. We will continue to add aliquots of ammonia until the solution is once again clear.

To complete the experiment you will now observe the spectrum of our fluorescent light source and the results of filtering that light through solutions containing either copper (II) sulphate or the copper-ammonia complex. Record changes in the intensity, location and colour of the bands or lines produced by the fluorescent light when filtered by the solutions.

Record the intensity, location and colour of the bands or lines produced by the fluorescent light.

Record changes in the intensity, location and colour of the bands or lines produced by the fluorescent light when filtered by the solution.