The thick black line encloses the seven to eight metals most often to often so recognized. Silver is sometimes not recognized as a noble metal on account of its greater reactivity.[4]
↑"Noble Metals, Analytical Chemistry of" in (2021) Encyclopedia of Analytical Chemistry: Applications, Theory and Instrumentation, Wiley Online Library DOI: 10.1002/9780470027318.a2411.pub3.
↑Rayner-Canham, G (2018) "Organizing the transition metals" in Mendeleev to Oganesson: A multidisciplinary perspective on the periodic table, Oxford University, pp. 195–205 ISBN: 978-0-190-668532.
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