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# 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.