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The Atomic Absorption Spectrum: what is it?An atomic absorption spectrum is a band of light with bits missing from it. The black lines are missing bits of light which have been absorbed by a sample of an element. Each element has its own particular absorption spectrum. These are known and fixed, and are always the same for the same element. How do we get an absorption spectrum?Light comes from a light source. That’s the bulb in the picture below. It is usually a pretty strong light source. A sample of an element, such as aluminium, chlorine etc, is vaporized. The light passes through the sample, and bits of the light are absorbed. The remainder of the light is then passed through a crystal, broken up into its component colours and then it can be analysed.
What is happening?The atoms start in their ground state which means they are just as you would find them naturally; they have not been given any extra energy and will stay in that state indefinitely. When the atoms are given energy and are able to absorb some of it, they are then said to be in an excited state. This just means they have more energy than in their ground state. The atoms in the sample are taking some of the energy in the light and hanging on to it. This energy is absorbed by electrons moving from one orbital, sub-shell or shell (see Schroedinger’s model for more explanation of these terms) to another. Electrons absorbing energy are moving FURTHER AWAY from the nucleus.
Just like a rocket leaving the earth, the electrons need to gain energy to move further away. In this case it is not gravity but electrical charge that is being overcome. The further the electrons move from the nucleus the more energy they need to do that. That’s why the shorter distance can be accomplished with red light (lowest energy in the visible spectrum) while the electron jumping two levels needs more energy, in the form of green light. Return from Atomic Absorption Spectrum to The Chemistry or return to the Green Planet home page for more Solar Power Facts.
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