Helium atomic radius is 31pm, how was this data obtained? The results of an online search are all 31pm. How was this data obtained? Is it measured or calculated?
Where are the reference links?
 A: What you are talking about is the 31pm, the calculated value from the center of the nucleus to the outermost electron shell.


Since the boundary is not a well-defined physical entity, there are various non-equivalent definitions of atomic radius. Depending on the definition, the term may apply only to isolated atoms, or also to atoms in condensed matter, covalently bound in molecules, or in ionized and excited states; and its value may be obtained through experimental measurements, or computed from theoretical models.


https://en.wikipedia.org/wiki/Atomic_radii_of_the_elements_(data_page)
Helium has the smallest such calculated atomic radii value.
https://periodictable.com/Properties/A/AtomicRadius.html


This data set is a set of theoretical atomic radii corresponding to the principal maximum in the radial distribution function, 4pr2r2 for the outermost orbital. THe data were calculated for the ground state of most elements of the periodic table using Slater orbitals.
    This gives calculated values of Rmax, the radius at which the magnitude of wave function is greatest, for the valence orbitals.


https://www.webelements.com/helium/atom_sizes.html


J.B. Mann, Atomic Structure Calculations II. Hartree-Fock wave functions and radial expectation values: hydrogen to lawrencium, LA-3691, Los Alamos Scientific Laboratory, USA, 1968.


This uses the calculation of atomic wavefunctions and energies using the self consistent field method of Hartree and Fock.
https://www.osti.gov/servlets/purl/4553157/
Please note that here you will find the value as 0.569176AU, which should be 0.31pm.
