The emitted or absorbed by atoms of certain elements



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An atomic spectrum is light the emitted or absorbed by atoms of certain elements. Atoms can only emit and absorb light of certain frequencies, and the frequency of light emitted or absorbed in the visible part of the spectrum can be seen when light is passed through a prism. The diagram below (1) shows an emission spectrum. The hot flame excites the atoms, and causes their energy to increase. It is when they lose energy and return to their ground state that they emit energy in the form of electromagnetic information (2).

This radiation is most often emitted in the infra red, visible or ultraviolet part of the spectrum. If the emitted light is then passed through a prism, it is split up into an atomic spectrum. Coloured lines will appear at certain intervals on the black background. Normally, there would be a continuous spectrum, but because atoms can only emit certain frequencies, the lines will only appear at those frequencies on the spectrum. For example, sodium emits yellow light in the visible part if the spectrum (2).

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In order for the emission spectrum to be seen, the atoms of an element must be in an excited state. However, in order for the absorption spectrum to be seen, the atoms must be their ground state. The diagram below (1) shows an absorption spectrum of an element. The flame is cooler, and most of the atoms are in their ground state. If white light is passed through this cooler sample, and this light is then passed through a prism, black lines will appear on a bright background.

These lines correspond exactly with the coloured lines in the emission spectrum of that specific element (2). For example, the yellow lines on the emission spectrum of sodium are is exactly the same place as the black lines on the absorption spectrum. As the sequence of lines in any atomic spectrum is characteristic only of the atoms in that element, it can be used to identify the element when that element is part of a compound or mixture. The intensities of the lines will also give an indication of the abundance of that element in the compound or mixture.

For example, if white light was passed through a cool sample of sodium chloride (table salt), the intensity of the black lines on the bright background would help to indicate the abundance of sodium in the compound. This can be carried out as an experiment in an ordinary laboratory. The top diagram on the left (2) shows the solar spectrum, and the diagram below that (2) shows how the continuous spectrum emitted by a hot body changes with temperature. Together, these diagrams show that the spectrum of the sun is around the same as that of a hot body a 6000K.

The sun appears yellow because it radiates most of its energy in the yellow part of the spectrum. The part of the sun that radiates energy is known as the photosphere (2). Whereas the emission spectrum of a hot body is continuous, the sun’s is not. In 1841, when Joseph Fraunhofer studied the solar spectrum, he found more than 600 dark lines on the otherwise continuous spectrum (2). These ‘Fraunhofer Lines’ show light that has been absorbed by particular atoms and ions. In particular Ca+ ions, Mg, Fe, Na and H atoms. However, this does not mean that the sun is made up solely of these 5 elements (2).

Most of the sun is made up of hydrogen (92%), and almost all of the remaining 8% is helium. There is hardly any calcium. The chemical composition of the sun is due to the different 1st ionisation enthalpies of the different elements. Hydrogen and helium have extremely high 1st ionisation enthalpies, while those of Ca, Fe, Mg and Na are extremely low. The temperature of the sun’s chromosphere is 4000K, and as the 1st ionisation enthalpy of hydrogen is almost twice that of calcium, it is logical to assume that there will be much less calcium than hydrogen (the actual ratio is 1:500,000 [2]).

Knowledge of the solar spectrum and the 1st ionisation enthalpies of elements can be used to determine the temperature of certain areas of the sun, and also the basics of its chemical composition. Interstellar clouds also emit radiation. However, the wavelength of this radiation is around 1 millimetre (3) – a much longer wavelength than visible light. This radiation is not absorbed by the earth’s atmosphere, but it can be observed with the use of radio telescopes. The telescopes can be used to observe the radiation in 2 ways:

Their detectors can be tuned to a particular frequency characteristic of a particular molecule. CO is abundant and is often used. The telescopes then search the night sky and map out the distribution of intensity at this frequency (3).  The telescopes can also be used to measure how intensity of radiation fluctuates with frequency. By scanning over a range of frequencies to which the detector is sensitive a spectrum can be recorded. Each peak can be attributed to a particular molecule by comparing the frequencies with spectra obtained in laboratory experiments. Abstract:

This report has examined emission and absorption spectra and has given examples of how this can be achieved in a laboratory; the chemical composition of the sun and interstellar clouds with regards to the solar spectrum and the use of radiotelescopes respectively; the rates of chemical reactions within interstellar clouds. (50 words) Specimen.

References: (1) Salters Advanced Chemistry – Chemical Ideas (Second Edition) – Heinemann 2000 (2) The Sun, Stars and Spectra – Charlie Harding – Chemistry Review – January 1992 (3) Molecules in Space – Ian Smith – Chemistry Review – January 1998 William Marney 7302.

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