Atomic Spectra – ‘fingerprints’ for elements

6 Maret 2010 pukul 05:04 | Ditulis dalam Uncategorized | Tinggalkan komentar

Electrons exist in energy states within the atom (called orbitals by chemists).

Generally, the further away from the nucleus these states are, the higher the potential energy of the electron in that state. When an electron is ‘free’ of the nucleus’ influence it is said to have zero potential energy – a zero energy state. When it is closer to the nucleus it has less potential energy (needs energy input to escape the influence). It is therefore at a negative energy level

We can put information about these energy states into a diagram

When an electron jumps between energy levels of an atom it either absorbs or emits a photon. The size of that photon’s energy depends on the energy gap between the levels. This is linked to its frequency by the equation

E = hf

Where

E is the energy of the photon absorbed or emitted (and the energy interval between the energy levels (in joule, to convert to eV divide by 1.6 x 10-19 )

h = Planck constant

f = frequency of the photon of electromagnetic radiation

(linked to its wavelength by c = f)

E = hf = hc/

Exciting the electrons

This can be done by bathing the gas sample in electromagnetic energy of a wide continuous spectrum (usually of visible light – but it is the same principle for other sections of the electromagnetic spectrum – only then you wouldn’t ‘see’ the result – you would need to use a detector). The electrons absorb the photons they need to make transitions to higher energy levels and then give them back out again when they return to the ground state. They only remain in an excited state for less than a microsecond and are therefore constantly absorbing/emitting photons from the electromagnetic source.

There are two type of atomic spectra. The graphic below shows the visible pectra for hydrogen. You can see that the lines are in the same place on both spectra – because they correspond to the same energy jumps.

Photons that do not have energies that correspond to the ‘gaps’ between energy levels for the electrons are not absorbed. They go through the gas onto the photographic plate (or other form of detector).

When they give out the photons they have absorbed they do so in random directions therefore only a tiny fraction of them are emitted in the direction they would have gone had they not been absorbed in the frst place. This results in a lower intensity of electromagnetic radiation of the wavelength that has been absorbed by the sample shining onto a photographic plate. We therefore get an absorption spectrum

Click here for an interactive activity page on absoption spectra.

Another way of exciting the electrons is electrically. By putting the gas in a discharge tube we can ‘zap’ the atoms and therefore their electrons with electrical energy. This promotes electrons from the ground state to excited states and then as they ‘jump down’ they emit the characteristic photons of the jumps within the electron orbital of the atom. These can be collected by a dispersion system such as a spectrometer (or prism) which will disperse them into wavelengths…. missing wavelengths will not appear on the film, gaps will be left. The only coloured lines will be those that correspond to the wavelengths of the photons that ‘fit’ the energy jumps between orbitals. The spectrum we see is called an emission spectrum.

Click here for an interactive activity page on emission spectra.

We can look at these spectra using a spectrometer. This is an instrument that used either prisms to disperse the light by refraction or diffraction gratings to disperse the light by diffraction

Check out this link and look at the spectra of various gases in dischage tubes

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