Elements#
element entries are needed only when defining custom elements that are not standard
chemical elements, or defining specific isotopes.
The fields of an element entry are:
- symbol
- The symbol used for the element, as used when specifying the composition of species. 
- atomic-weight
- The atomic weight of the element, in unified atomic mass units (dalton). 
- atomic-number
- The atomic number of the element. Optional. 
- entropy298
- The standard molar entropy of the element at 298.15 K. Optional. 
An example elements section:
elements:
- symbol: C13
  atomic-weight: 13.003354826
  atomic-number: 6
- symbol: O-18
  atomic-weight: 17.9991603
