Charge radius

The charge radius of an atomic nucleus tells its size. The nucleus (center) of an atom is incredibly tiny. A nucleus, as with an atom, is actually a hazy quantum-mechanical wave-cloud, because each of the particles inside it are also hazy wave-clouds. Not having a definite surface, the cutoff is usually statistical, and can be calculated in a variety of ways.

Name	typical or avg size	refs
atomic nucleus	0.003 pm
atom	100 pm
light wavelength	550 000 pm
human cell nucleus	6 000 000 pm
human HeLa cell	16 000 000 pm
one inch	25 400 000 000 pm

The rms charge radius is a measure of the size of an atomic nucleus, particularly the proton distribution. The proton radius can be measured by the scattering of electrons by the nucleus. Relative changes in the mean squared nuclear charge distribution can be precisely measured with atomic spectroscopy.