PrIMe Chemical Elements records contain basic information on chemical element, as symbol,
element number, relative atomic mass, and information on element's isotopes - mass and relative abundance.
Chemical element records are designated by primeID identifier "e", followed by an 8-digit sequence. Differently
from all other XML files in the PrIMe Warehouse, chemical element files are named not by primeID, but by element's symbol.
For every numerical value uncertainty can be specified.