This constant, `r_d`, defines the rms charge radius of deuteron, the nucleus of Deuterium.  This constant for the rms charge of Deuteron is expressed in meters (m).

Notes

Deuterium (symbol D or 2H, also known as heavy hydrogen) is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in 6,420 of hydrogen.

This constant, `r_d`, is specified with a standard uncertainty (standard deviation) of: 0.0021 E-15 m

The root mean square (rms) charge radius is used to measure the size of an atomic nucleus.  The distance is computed as a root mean square (rms) value because atoms and nuclei have no definite boundaries and thus it is appropriate to calculate a statistical model of the sphere of charge seen by the electrons.  The dimension of the charge effect which represents the size of the nuclei is derived from electron scattering.