Noble Metals

What Are Noble Metals?

Platinum is an example of a noble metal.
Platinum is an example of a noble metal. Periodictableru

You may have heard certain metals called noble metals. Here is a look at what the noble metals are, which metals are included and the properties of the noble metals.

What Are the Noble Metals?

The noble metals are a group of metals that resist oxidation and corrosion in moist air. The noble metals are not easily attacked by acids. They are the opposite of the base metals, which more readily oxidize and corrode.

Which Metals Are Noble Metals?

There is more than one list of noble metals. The following metals are considered noble metals (listed in order of increasing atomic number):

  • ruthenium
  • rhodium
  • palladium
  • silver
  • osmium
  • iridium
  • platinum
  • gold

Sometimes mercury is listed as a noble metal. Other lists include rhenium as a noble metal. Strangely, not all corrosion-resistant metals are considered to be noble metals. For example, although titanium, niobium and tantalum are extremely corrosion-resistant, they are not noble metals.

While acid resistance is a quality of noble metals, there are difference in how the elements are affected by acid attack. Platinum, gold, and mercury dissolve in the acid solution aqua regia, while iridium and silver do not. Palladium and silver dissolve in nitric acid. Niobium and tantalum resist all acids, including aqua regia.

Calling a metal "noble" may also be used as an adjective to describe its chemical and galvanic activity.

Under this definition, metals may be ranked according to whether they are more noble or more active. This galvanic series can be used to compare one metal with another for a particular application, typically within a set of conditions (such as pH). In this context, graphite (a form of carbon) is more noble than silver.

Physics Definition of Noble Metals

Chemistry allows for a loose definition of noble metals, but the physics definition is more restrictive. In physics, a noble metal is one which has filled electronic d-bands. According to this definition, only gold, silver and copper are noble metals.

Uses of the Noble Metals

Generally speaking, the noble metals are used in jewelry, coinage, electrical applications, to make protective coatings, and as catalysts. The exact uses of the metals vary from one element to another.

Platinum, Gold, Silver, and Palladium: This are bullion metals, used to make coins and jewelry. These elements are also used in medicine, particularly silver, which is antibacterial. Because they are excellent conductors, these metals may be used to make contacts and electrodes. Platinum is an excellent catalyst. Palladium is used in dentistry, watches, spark plugs, surgical instruments, and as a catalyst.

Rhodium: Rhodium may be electroplated over platinum, sterling silver, and white gold to add shine and protection. The metal is used as a catalyst in the automotive and chemical industries. It's an excellent electrical contact and can be used in neutron detectors.

Ruthenium: Ruthenium is used to strengthen other alloys, particularly those involving other noble metals.

It's used to make fountain pen tips, electrical contacts, and as a catalyst.

Iridium: Iridium is used in many of the same ways as ruthenium, as both metals are hard. Iridium is used in spark plugs, electrodes, crucibles, and pen nibs. It's valued for making small machine parts and is an excellent catalyst.

View the Chart of Noble and Precious Metals.