WordNet (r) 3.0 (2006):

chemical element
    n 1: any of the more than 100 known substances (of which 92
         occur naturally) that cannot be separated into simpler
         substances and that singly or in combination constitute all
         matter [syn: chemical element, element]

Moby Thesaurus II by Grady Ward, 1.0:

67 Moby Thesaurus words for "chemical element":
   acid, acidity, agent, air, alkali, alkalinity, alloisomer, anion,
   antacid, atom, atomic particles, base, biochemical, brute matter,
   building block, cation, chemical, chromoisomer, component,
   compound, constituent, copolymer, dimer, earth, element,
   elementary particle, elementary unit, fire, fundamental particle,
   heavy chemicals, high polymer, homopolymer, hydracid, hyle,
   hypostasis, inorganic chemical, ion, isomer, macromolecule,
   material, material world, materiality, matter, metamer, molecule,
   monad, monomer, natural world, nature, neutralizer, nonacid,
   organic chemical, oxyacid, physical world, plenum, polymer,
   pseudoisomer, radical, reagent, stuff, substance, substratum,
   sulfacid, the four elements, trimer, unit of being, water