Data Sources

Physical constants
NIST Physics Laboratory - Constants, units and uncertainty
Atomic and isotope mass
NIST Physics Laboratory - Atomic weights and isotope composition
Atomic density
ILL Neutron Data Booklet
Covalent Radii
Cordero, et al., Dalton Trans., 2008, 2832-2838, doi:10.1039/801115j
Magnetic form factors
Brown. P. J., In International Tables for Crystallography, Volume C, Wilson. A. J. C (ed), section 4.4.5
Neutron scattering factors
Atomic Institute for Austrian Universities
X-ray scattering factors
Center for X-Ray Optics
Crystal structure
Ashcroft and Mermin
Oxidation states

wikipedia: List of oxidation states of the elements

Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. pp. 27–28. ISBN 0-08-037941-9.

FASTA and biomolecule formulas
Perkins, Modern Physical Methods in Biochemistry Part B, 143-265 (1988)