A simplified representation of a helium atom, the second smallest of all atoms, having an estimated (calculated) radius of 31 picometres^[1]

A picometre (symbol pm) is a unit of length in the metric system, equal to one trillionth, i.e. (¹/_{1,000,000,000,000}) of a meter, which is the current SI base unit of length. It can be written in scientific notation as 1×10⁻¹² m (scientific notation) or 1 E−12 m (engineering notation) — both meaning 1 m / 1,000,000,000,000.

It equals a millionth of a micrometre (formerly called a micron), and was formerly called micromicron, stigma, or bicron.^[2] The symbol µµ was once used for it.^[3] It is also a hundredth of an Ångström, an internationally recognized non-SI unit of length.

The picometre's length is of an order such that its application is almost entirely confined to particle physics and quantum physics. Atoms are between 62 and 520 pm in diameter. Smaller units still may be used to describe smaller particles (some of which are the components of atoms themselves), such as hadrons and fermion point particles.

[edit] References

1. ^ "Atomic radius". WebElements: the periodic table on the web. http://www.webelements.com/periodicity/atomic_radius. 
2. ^ Deza, Elena; Deza, Michel Marie (2006). Dictionary of Distances. Elsevier. ISBN 0444520872. http://books.google.com/books?id=I-PQH8gcOjUC&pg=PA347&dq=stigma+bicron&lr=&ei=tYjISJz5HaaktAPJkqjkBQ&sig=ACfU3U3U2S3gikmIt4r9RO8dM3GYllujSA. 
3. ^ How Many? A Dictionary of Units of Measurement; Russ Rowlett and the University of North Carolina at Chapel Hill; http://www.unc.edu/~rowlett/units/dictB.html