Atomic spectroscopy

Atomic spectroscopy is the technique of analyzing the photons emitted by atoms in order to determine the elemental composition of a substance.

Electrons can exist only at certain discrete energies. These energies are characteristic of each element; that is, every isolated atom of an element has the same set of available energies. Normally, electrons in atoms are distributed in the lowest energy levels. This is called the “ground state” of the atom. If energy is added to the atom in the form of light, heat, or electricity, the electrons can move to a higher energy level. The electrons are said to be in an “excited” state. When the electrons return to their ground state distribution, they emit the excess energy in the form of light. The light carries the energy that is the difference between one energy level and another. The distribution of light energies is called a spectrum (plural spectra). The study of spectra is called spectroscopy. When light is emitted from an atom, the different colors of light can be seen when they are separated by a device such as a prism or a diffraction grating. If white light passes through a prism or grating, you see a full rainbow, but when the light emitted by an element passes through, not every color is there—only those specific colors corresponding to energy level differences. These correspond to lines in the spectrum.

A line spectrum is very useful in identifying an element because no two elements have the same line spectrum. This is how elements can be identified even on far away stars. The spectrum of light from the star is analyzed for the lines of color in its spectrum. These lines can then be matched to known line spectra of elements on Earth. The element helium was discovered in this way. Its line spectrum was seen when sunlight was passed through a prism.

See also Electron.