Electrons of an atom exist in well defined energy levels which can jump to higher energy levels on absorbing thermal or electrical energy. The atomic absorption is subsequently followed by emission of light radiation. The wavelength of this emitted radiation is distinct and characteristic of every atom. With the help of a spectrometer the wavelength of these electromagnetic radiations can be visualized and measured. The emission lines in all other series for the atom can then be calculated using the Balmer-Rydberg equation. This establishes an inter-connection between the electronic transitions and the corresponding wavelengths which can be depicted in the form of graphs.
Atoms emit light radiations when energy in the form of electricity or heat is supplied. In the ground state, the electrons of an atom exist in energy levels with well defined energies. When atoms absorb energy, their electrons get excited and move to the higher energy shells. This process is known as atomic absorption. In the process they release light energy leading to the process of electron emission. This emitted radiation comprises of light rays of distinct wavelengths lying in different regions of the electromagnetic spectrum.
The wavelength of the radiation emitted by the electron bears direct connection to the electronic transition. The complex nature of the electronic configuration of an atom can result in multiple electronic transitions. Furthermore, since every element has a unique electronic structure and therefore the wavelengths of the emitted radiations follows a distinct pattern. The sequence of emitted radiations when recorded gives rise to a series of discrete emission lines separated at certain intervals.
Each spectral line corresponds to the wavelength of the emitted radiation. The instrument known as Spectrophotometer, measures the intensity of the emitted radiation as a function of wavelength.
Spectroscopy is a process by which the atomic properties of an element can be determined. The aim of the experiment is to build a simple spectroscope which can be used in the case of the simple atoms to observe their line spectra lying in the visible region of the electromagnetic spectrum.
This spectroscope can be used to carry out quantitative analysis in the following applications:-
- Identification of the components of a sample.
- Measurement of the amounts of the components
- Preparation of a simple spectroscope and using it to observe the line spectra of the of simple atoms in the visible region of the electromagnetic spectrum.
Materials Required: Empty cardboard box, Razor blades or Index Cards, Diffraction grating and a Graph Paper.
- Mark two straight lines at an angle of 30o with each other. The green portion ranging to 500nm of the visible spectrum is placed near the middle of the viewing region. Insert holes at the two ends of the slanting line. One end of this line acts as a diffraction grating while the other is used for gas exchange.
- The groove separation of the grating system is kept at 1000nm i.e. 1000 lines/mm transmission grating.
- Make a slit using razor blades as wide as 1mm for better resolution. A 2mm gap (1/2” by 3/4") will ensure better spectra.
- Mark numbered grids also to the outside of the box. Put the graph paper where the spectrum will be project (Grossie, 2010).
- Calculation of the wavelengths associated with the emission lines and establishing their relationship to the electronic transitions within an atom.
The Spectroscope has to be used to observe and record emission lines of helium. By plotting a graph between the wavelengths the readings on the graph paper will generate a proportion that will help determine the wavelengths of other spectral lines.
The wavelengths corresponding to more complex electronic transitions can be calculated using the Balmer-Rydberg equation. According to the Balmer-Rydberg equation, we have:-
Where ‘m’ and ‘n’ are the principle quantum numbers of the upper and lower principle shells respectively and λ represents the wavelength (Grossie, 2010).
Where R=109678 cm-1.
Fig. 1 Graph plotted between wavelength and scale reading in a Helium atom
The results obtained from the experiments can be summed up as follows.
- The wavelengths continually increase from violet to red, thus showing a straight line in the graph.
- The error-rate is highest in the blue-violet region of 13.388% while the least error-rate 1.77% is found in the red region.
- Neon gives the brightest and maximum emission lines while the emission lines of Argon and Krypton will be dimmer and fewer.
A simple spectroscope can prove to be a very useful tool in determining the atomic properties of an element. With the help of this device, we can find out the wavelength corresponding to the electronic transitions that lie in the dominant regions. The scale readings obtained at the Spectroscope gives a clear idea about the proportional relationship between the wavelengths. The Balmer-Rydberg equation can then applied to calculate the remaining wavelengths. Since, every element has a characteristic atomic nature, the spectral lines will bear distinct patterns in every case.
The experiment was conducted successfully and the desired results were obtained. The error-rate had been considerably low and no gross errors were observed. The wavelengths of the subsequent radiations have been calculated with a good degree of precision.
“Atomic Spectroscopy.” Andor Technology. andor.com. N.p., n.d. Web. 31 Mar. 2013.
“Atomic Spectra.” Spectroscopy. chemistry.tutorvista.com. N.p., n.d. Web. 31 Mar. 2013.
Grossie, D.A. and Underwood, K.A. “Spectroscopy and Atomic Spectra.” Laboratory Guide for Chemistry. London: Pearson Prentice Hall, Inc, 2010. Print.