picometer

The picometre is the chemist's ruler. Bond lengths land in the low hundreds of picometres: 154 pm for a carbon-carbon single bond, 96 pm for the O-H bond in water, 74 pm between the two atoms of the hydrogen molecule. The Bohr radius, 52.918 pm, sets the natural size of the hydrogen atom and sits among the constants this site carries, so atomic structure calculations land in this unit whether or not anyone asked for it.

Crystallography hides a pleasing coincidence at this scale. The workhorse X-ray line of diffraction laboratories, copper K-alpha, has a wavelength of 154.2 pm, almost exactly the length of the carbon-carbon bond it is so often used to resolve. The near-equality is pure luck, but it illustrates the requirement that makes X-ray diffraction possible at all: resolving atomic spacings takes a wavelength of comparable size, and visible light, at around 500,000 pm, is hopeless by three orders of magnitude. The practical trap is the decimal shift against the older unit flagged in the caution: crystal structures published in ångströms convert at exactly 100 pm per ångström, so a 1.54 Å line and the 154 pm line are the same thing, and misplacing that factor of 100 puts a bond length on the scale of an atomic nucleus or a virus.