Connelly, N. G., Royal Society of Chemistry (Great Britain) e International Union of Pure and Applied Chemistry., Nomenclature of inorganic chemistry. IUPAC recommendations 2005, Royal Society of Chemistry,, 2005, p. 10, ISBN 0854044388, OCLC 60838140. URL consultato il 23 ottobre 2018.
«The problem is to define the term ‘metal’.»