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The picometre or picometer (American spelling) (international spelling as used by the International Bureau of Weights and Measures; SI symbol: pm) is a unit of length in the metric system, equal to 1×10−12 m, or one trillionth (US system) of a meter, which is the SI base unit of length.
The picometre's length is of an order so small that its application is almost entirely confined to particle physics, quantum physics, chemistry and acoustics. Atoms are between 62 and 520 pm in diameter, and the typical length of a carbon-carbon single bond is 154 pm. Smaller units still may be used to describe smaller particles (some of which are the components of atoms themselves), such as hadrons and the upper limits of possible size for fermion point particles.