What's the "een" in Halloween?
before vowels chem-, word-forming element denoting "relation to chemical action or chemicals," from comb. form of chemical (adj.), used to form scientific compound words from c.1900.
chemo che·mo (kē'mō, kěm'ō)n. Chemotherapy or a chemotherapeutic treatment.
chemo- or chemi- or chem-pref. Chemicals; chemical: chemotherapy.