As ChemDataExtractor processes documents, it adds each unique word that it encounters to the Lexicon as a Lexeme.
Each Lexeme stores various word features, so they don't have to be re-calculated for every occurrence of that word.
You can access the Lexeme for a token using the lex property.
>>> s = Sentence('Sulphur and Oxygen.')
>>> s.tokens[0]
Token('Sulphur', 0, 7)
>>> s.tokens[0].lex.normalized
'sulfur'
>>> s.tokens[0].lex.is_hyphenated
False
>>> s.tokens[0].lex.cluster
'11011101100110'