Spectroscopy Construction and Atomic Spectra

Abstract

Light is emitted when atoms are heated at high temperatures. This light is characteristic for the element being heated. A spectroscope separates constituents of light and can be used to measure the wavelengths of different elements based on the color observed. A spectroscope, which is an essential tool in analytical chemistry can be made in the laboratory using simple materials and be used to measure the wavelengths of light emitted or absorbed by different elements. A cake mix box was used to make a simple spectroscope that was used to establish the wavelengths of five different elements (hydrogen, helium, argon, krypton, and neon) with the aid of a calibration curve. It was realized that the observed wavelengths were almost similar to the documented literature values of the given elements. It was concluded that spectroscopy was a reliable technique of establishing the wavelengths of chemical elements and the subsequent identification of these elements.

Introduction

Spectroscopy is a field of science that deals with the investigation and quantification of spectra produced as a result of matter’s interaction with light or electromagnetic radiations. The heating of ions and atoms to elevated temperatures results in the emission of light. When this light is passed through a spectroscope, it reveals a succession of colored lines. These lines are distinctive for the type of element. It is possible to separate light into its constituents. For example, a prism can break white light into components of the visible light spectrum.

Spectral lines form when electrons shift from one energy level to another in the atom’s outermost energy shell. Bohr’s representation of the atom is used to explain the formation of atomic spectra (“Atomic Spectra” 7). There are three types of spectra namely the emission line, the absorption line and the thermal spectra. Bound electrons have the ability to absorb or give off energy only in precise distinct quantities. An electron leaps to an advanced energy level when it takes up a photon of light with the exact quantity of energy leading to the formation of an absorption spectrum. On the other hand, after the electron reverts to a lower energy state a photon is given off leading to the formation of an emission spectrum. Therefore, when many electrons jump between two energy levels in a given element, emission or absorption spectra are produced at characteristic wavelengths. Closely packed atoms give rise to a thermal spectrum where the neighboring atoms’ electrons disfigure the energy levels of these atoms. Consequently, the typical sharp spectral lines spread out.

A spectroscope is an instrument that breaks up light into distinctive wavelengths. Light gets into the spectroscope through a slit, which is set at an angle. A collimator then makes the light beam parallel before reflecting it off to the diffraction grating. The light is then scattered into its constituent wavelengths, colors or energies by the diffraction grating. Every wavelength emerges from the grating at an altered angle. The beam of light is subsequently directed and its image is formed using a camera lens. An eyepiece can similarly be used to perform the function of image formation.

The equation used to establish the correct wavelength is 1/λ=R (1/m2-1/n2) (1) where m and n are integers representing the higher and lower energy levels and R is Rydberg constant (1.0968×107m-1). A calibration plot is necessary when determining the correct wavelength because it caters for the correlation between the response of the instrument and the analytes being measured. In addition, a calibration curve gives the limits of detection of the instrument thereby enhancing accuracy and precision of an experimental procedure.

The aim of this investigation was to construct a simple spectroscope and use it to study the visible line spectra of five different atoms. It also aimed at relating the observations to electronic transitions within the atoms.

Procedure

A cake mix box at least one inch deep was used to make the spectroscope. A slot one inch deep and a quarter inch wide was made on the box in such a manner that light passing through the slot made an angle of 30o between the eyehole (diffraction grating) and a graph paper scale (normal incidence). Two razor blades were fastened together to narrow the entrance of the slot so that it was less than a millimeter wide. The gap between the two razor blades formed the light entrance. It was ensured that the instrument was well aligned to prevent the formation of a superimposed slit image on the graph paper scale. An ordinary tungsten light bulb was used to match the spectrum to the scale. Spectral lines were seen to be superimposed on the graph paper scale. Several graph paper lines were darkened and were used to calibrate the spectroscope.

A helium discharge tube was used to calibrate the spectroscope. This was achieved by establishing the spectral lines corresponding to the reference lines. Tables containing columns for the scale reading, the wavelength and the detected color were made. The helium data were used to plot a five-point graph with the scale reading on the x axis and the wavelength on the y axis. The best line of fit was made, which was used as the calibration line in the establishment of other spectral lines from their corresponding scale readings.

The same procedure was used to measure the positions and intensities of lines of the spectra of neon, argon and krypton (using the helium calibration curve). Comparisons were made between these values and the literature values.

Data and Results

Table 1: Experimentally Observed Helium Spectrum Lines

(nm) Scale reading Color
667.8 51 Red
587.6 45 Yellow
501.6 25 Green
492.2 24 Blue-green
471.3 20 Blue
438.8 18 Violet

The wavelengths were obtained by observing the colors within the visible light spectrum that could be seen with the naked eye. These colors were matched with the wavelengths in the visible light spectrum.

Helium Calibration Plot
Figure 1: Helium Calibration Plot

The above figure was obtained by plotting the wavelengths in nm against scale readings (mm) using Microsoft Excel. The equation on the plot showed the relationship between the wavelength and the scale readings, which was used in the determination of wavelengths for other elements.

Table 2: Experimentally Observed Hydrogen Spectrum Lines

Exp. (nm) Scale reading Color
613.5 45 Red
523.5 30 Green
468.5 21 Blue-violet
409.5 11 Indigo

Table 3: Calculated Hydrogen Spectrum Lines

m N Calc. (nm) Exp. (nm) Error % (calc. –exp)
2 3 656.49 613.5 6.54
2 4 486.17 523.5 7.61
2 5 434.08 468.5 7.93
2 6 410.21 409.5 0.173

Sample calculation when m=2 and n=3

From the Rydberg equation 1/λ=R (1/22-1/32)

1/λ=R (1/4-1/9)

1/λ=R (0.13888)

1/λ=1,523,235.84

λ=1/1,523,235.84

λ=6.5649×10-7m-1

=656.49 nm.

Table 4: Experimentally Observed Neon Spectrum Lines

Exp. (nm) Scale reading Color
607.5 44 Red
583.5 40 Orange
559.5 36 Yellow
523.5 30 Green
463.5 20 Blue

The scale readings were obtained from the spectroscope after which the helium calibration curve was used to compute the values of the wavelengths (by substituting the scale reading for x in the equation y=6.001x+343.5).

Table 5: Experimentally Observed Argon Spectrum Lines

Exp. (nm) Scale reading Color
667.5 54 Red
559.5 36 Orange
517.5 29 Green
457.5 19 Blue

The scale readings were obtained from the spectroscope after which the helium calibration curve was used to compute the values of the wavelengths (by substituting the scale reading for x in the equation y=6.001x+343.5).

Table 6: Experimentally Observed Krypton Spectrum Lines

Exp. (nm) Scale reading Color
559.5 36 Yellow- orange
529.5 31 Green
433.5 15 Violet

The scale readings were obtained from the spectroscope after which the helium calibration curve was used to compute the values of the wavelengths (by substituting the scale reading for x in the equation y=6.001x+343.5).

Discussion

The observed wavelengths in the hydrogen spectra were part of the Balmer series. The experimental values were within the range of documented literature values of the elements (Lide 10-3). However, there were slight variations in the values obtained from the experiment because of experimental errors. Those variations were probably because of variations in standard laboratory conditions during the experiment and during the derivation of the literature values. In addition, literature values were likely to be averages of several experimental trials, whereas the experimental values were obtained from only one trial. Another reason for the differences between the literature values and experimental values was the nature of the spectroscope. The experiment used a makeshift spectroscope made from simple materials, whereas there were high chances that the literature values were obtained from standardized instruments made to achieve the highest levels of accuracy and precision. Therefore, it was likely that the spectroscope introduced errors in the measurements.

All physical measurements are bound to have errors, which can either be random or systematic. Systematic errors are partialities in measurements that cause the mean of several measurements to vary greatly from the actual measurements. Systematic errors arise mainly when the measurement instruments are faulty or due to improper handling of the instruments. In addition, wrong calibration of instruments can cause systematic errors. Random errors, on the other hand, are consequences of unpredictable modifications in the experimental environment. The errors observed in this experiment were probably a mixture of both types of errors because of the inconsistency in the margins of the percentage errors. Three of the observed wavelengths had relatively high percentages of errors (6 to 7%), whereas one measurement had a very small margin of error (0.17%). The high levels of errors were probably because of faults in the instruments. It was possible there was an improper alignment of the instrument. It was also possible that the razor blades in the slot had a large space between them leading to the production of blurred spectral lines. It was also possible that there were inconsistencies in the handling of the instrument.

Conclusion

Spectroscopy is an essential technique in many sciences such as chemistry, biochemistry, physics and astronomy. Though the experimental results varied slightly from the actual literature values, there were high similarities between the two sets of readings. Therefore, it could be concluded that the makeshift spectroscopes were reliable tools in the establishment of wavelengths of different elements.

Works Cited

Atomic Spectra n.d. 2013.

Lide, R. David 2005, CRC Handbook of Chemistry and Physics, Internet Version. PDF file. 2013. Web.