Noble metals

Noble metal

Scatter plot of electronegativity values and melting points for metals (up to fermium, element 100) and some borderline elements (Ge, As, Sb). Those recognised by most chemists as noble metals have relatively high electronegativity,^[6] while their melting points show a distinction between silver and gold at around 1000 °C (~ 1275 K) compared to around 1500 °C (~1775 K) for palladium, the most volatile of the platinum group metals.^[7] While tungsten has a high electronegativity and a high melting point it is oxidised above 350 °C, and attacked by fluorine at room temperature.^{[citation needed]}

The geography of the plot broadly matches that of the periodic table. Starting from the bottom left, and proceeding clockwise, the alkali metals are followed by the heavier alkaline earth metals; the rare earths and actinides (Sc, Y and the lanthanides being here treated as rare earths); transition metals with intermediate electronegativity values and melting points; the refractory metals; the platinum group metals; and the coinage metals leading and forming a part of the post-transition metals.
In chemistry, noble metals are metallic elements that show outstanding resistance to chemical attack even at high temperatures.^[8] They are well known for their catalytic properties and associated capacity to facilitate or control the rates of chemical reactions.^[8] The short list of chemically noble metals (those elements upon which almost all chemists agree)^{[citation needed]} comprises ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), platinum (Pt), gold (Au), silver (Ag).^[9] In periodic table terms, an analogy can be made between the noble metals and the noble gases, which are mainly unreactive.^[10]
More inclusive lists include one or more of copper (Cu), rhenium (Re), and mercury (Hg) as noble metals.

Noble metals in the periodic table

Elements categorised as such^[1]   Also recognised by (Arb) Brooks^[2]   Arb Ahmad^[3]   Arb Wells^[4]   Arb Tamboli et al.^[5]

Elements commonly recognised as metalloids   Noble gases

While noble metal lists can differ, they tend to cluster around the six platinum group metals—ruthenium, rhodium, palladium, osmium, iridium, and platinum, and gold.
In addition to this term's function as a compound noun, there are circumstances where noble is used as an adjective for the noun metal. A galvanic series is a hierarchy of metals (or other electrically conductive materials, including composites and semimetals) that runs from noble to active, and allows one to predict how materials will interact in the environment used to generate the series. In this sense of the word, graphite is more noble than silver and the relative nobility of many materials is highly dependent upon context, as for aluminium and stainless steel in conditions of varying pH.^[11]
The term noble metal can be traced back to at least the late 14th century^[12] and has slightly different meanings in different fields of study and application.
Prior to Mendeleev's publication in 1869 of the first (eventually) widely accepted periodic table, Odling published a table in 1864, in which the "noble metals" rhodium, ruthenium, palladium; and platinum, iridium, and osmium were grouped together,^[13] and adjacent to silver and gold.