ChemicalFormula

ChemicalFormula[<|elem1->n1,elem2->n2,…|>] represents a chemical species with ni atoms of the element elemi.

ChemicalFormula[chem] returns the chemical formula corresponding to the given input.

ChemicalFormula[…,<|qual1->val1,qual2->val2 …|>] represents a species whose qualifiers quali have values vali.

Please visit the official Wolfram Language Reference for more details and examples on core symbols.