atomic spectrum
/ uh-TOM-ik SPEK-trum /
An atomic spectrum is an element's fingerprint written in light. Where a rainbow from sunlight is a smooth, continuous smear of colour, the light from a single hot element is just a few sharp, isolated lines at very specific colours — a barcode unique to that element and no other.
More precisely, an atom's electrons can only sit at certain fixed energy levels. When an electron jumps between two levels, it absorbs or emits light of one exact wavelength set by the gap between them. Because each element has its own ladder of levels, it produces its own particular set of lines — its atomic spectrum.
It matters because this is the whole reason atomic spectroscopy works: which lines appear tells you which element is present (qualitative), and how bright or how deeply absorbed a line is tells you how much (quantitative). The catch is that complex elements with many electrons, like iron or the rare earths, throw thousands of lines that crowd together and breed spectral interference.
Pass light from a sodium lamp through a prism and instead of a rainbow you see two bright yellow lines very close together near 589 nm — sodium's famous doublet, its unmistakable spectral signature.
A few sharp lines, not a continuous rainbow: each element's own barcode.
An atomic spectrum is a line spectrum — discrete lines from free atoms. Contrast it with a molecular spectrum, which shows broad bands because molecules also store energy in vibrations and rotations, smearing the sharp lines into wide humps.