What are Noble Metals Definition Characteristics and Examples
Noble metals meaning refers to the selection of the periodic table's transition metal group. The list includes Silver, Gold, Platinum, Iridium, Palladium, Rhodium, Ruthenium, and Osmium. These metals are used to make noble alloys and various valuable elements. The noble metal prices vary hugely depending on their properties and various other factors like the ease to find and extract them and the noble metal refining process.
This section contains a brief description of the noble metals and popular 9 noble metals along with the others.
A Noble Metal: Brief Explanation
Platinum also refers to a subset of some of these elements, famous as the Platinum group and Ruthenium, Rhodium, Palladium, Platinum, Iridium and Osmium. Furthermore, a noble metal can also be referred to as an inert metal, and it can be a more or a less noble metal. Ruthenium, Rhodium, Iridium and Osmium are also famous as refractory metals defined by having melting points above 2000°C.
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Most noble metals crystallise in the fcc (face-centred cubic) structure, except Ruthenium and Osmium. These three metals have an hcp (hexagonal close-packed) structure. A noble metal includes the properties at the nanoscale, and its applications are mainly in the fields of biomedicine and catalysis.
Silver (Ag)
Silver has excellent electrical conductivity and higher chemical stability. Bulk Silver acts as a common element for making the higher-quality reflectors of electromagnetic radiations in a visible region, thus superseding the cheaper non-noble metals. Ag NPs have various applications like antimicrobial actions and catalytic properties, including enhancing the drinking water's microbial quality. Ag NPs represent ferromagnetic behaviour, and the percentage concentration of its surface atoms increases with the decrease in its particle size.
Gold (Au)
Due to exceptional chemical stability and good electrical conductivity, this noble metal has various significant uses. It is one of the best optical reflectors for the infrared region of the electromagnetic spectrum. Gold NPs have a more substantial size-dependent position of localised Plasmon resonance and have acquired attention in biomedicine, electronics and catalysis. Gold NPs also have a size-dependent magnetic behaviour that originates from the electron transfers between Gold's surface atoms and capping agents.
Platinum (Pt)
It is also ductile and malleable but is denser than the other ones. This inert metal is also chemically unreactive, but it is known to have a more significant density of free electrons, making it a suitable chemical catalyst. Platinum is widely used in catalytic converters for oxidising carbon monoxide that generates in combustion engines. Platinum also serves as a versatile electrode for electrochemical experiments. Platinum NPs are mainly engineered for catalytic applications and is also known to have been helpful in cancer therapies.
Palladium (Pd)
Palladium is a silvery-white metal found in deposits with Platinum, Nickel, and Copper. It has corrosion resistance, and its noble alloys are used for making jewellery as "white gold." It has an extensive capacity for absorbing hydrogen in a ration 900:1 by volume, and thus it serves as an excellent catalyst for hydrogenation and dehydrogenation reactions. There are various ways to synthesise Palladium NPs, and they have various applications like antimicrobial actions and enhanced Raman scattering.
Iridium (Ir)
It is also a silvery-white metal with higher corrosion resistance and higher density. It is an unworkable metal, but it is used in space components and spark plugs when alloyed with Platinum. Iridium NPs are synthesised using various chemical methods and tested as catalysts and sensors. It also helps as a biosensor to detect glucose.
Osmium (Os)
Osmium is a member of the Platinum group and is found in its ores. It has a higher density and melting point. The alloys of osmium and Platinum are harder than Platinum and are helped for speciality equipment. Osmium oxide is toxic to the respiratory system. Osmium NPs are used for hydrogenation reactions, CO oxidation, and electrocatalysis for PEM fuel cells.
Ruthenium (Ru)
Ruthenium is chemically inert and has a silvery colour. With less than 1% concentration, Ru can raise Palladium and Platinum alloys' hardness and increases the corrosion resistance in Titanium. Ruthenates also appear in electronics as the thicker film resistors, and some appear in the explorations of magnetism, superconductivity, and multiferroic prototypes.
Rhodium (Rh)
Rhodium is a harder and silvery-white transition metal with corrosion resistance and chemically inert action. It is extensively used in the automotive industry and has other applications, like neutron flux detectors and electrical contracts. It also serves biological needs and heterogeneous catalysis.
FAQs on Noble Metals in Chemistry Properties and Applications
1. What are noble metals in chemistry?
Noble metals are metals that are highly resistant to oxidation and corrosion under normal environmental conditions. They do not readily react with oxygen, water, or most acids.
- Common noble metals include gold (Au), silver (Ag), platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir), osmium (Os), and ruthenium (Ru).
- Their chemical stability is due to their filled or nearly filled d-orbitals and high ionization energies.
- They are often found in their native (elemental) state in nature.
2. Which elements are considered noble metals?
The noble metals are Au, Ag, Pt, Pd, Rh, Ir, Os, and Ru, mainly located in the transition metals of the periodic table.
- Gold (Au) and silver (Ag) are coinage metals.
- Platinum group metals (PGMs) include Pt, Pd, Rh, Ir, Os, and Ru.
- They are grouped together because of their low chemical reactivity and high resistance to corrosion.
3. Why are noble metals so unreactive?
Noble metals are unreactive because they have high standard reduction potentials and stable electron configurations.
- For example, the standard reduction potential of Au3+ + 3e- → Au(s) is +1.50 V, indicating a strong tendency to remain reduced.
- They resist oxidation by oxygen and water under normal conditions.
- Their filled d-subshells contribute to chemical stability.
4. What is the difference between noble metals and precious metals?
Noble metals are defined by their low chemical reactivity, while precious metals are defined by their economic value and rarity.
- Most noble metals, such as Au and Pt, are also precious metals.
- However, a metal can be precious due to rarity but not necessarily extremely unreactive.
- The term “noble” refers to chemical behavior, whereas “precious” refers to market value.
5. Do noble metals react with acids?
Most noble metals do not react with common acids, but some dissolve in aqua regia, a mixture of concentrated HCl and HNO3.
- Gold reacts with aqua regia forming chloroauric acid: Au(s) + 3HNO3(aq) + 4HCl(aq) → HAuCl4(aq) + 3NO2(g) + 3H2O(l).
- Platinum also dissolves in aqua regia.
- Silver reacts with nitric acid but not with hydrochloric acid alone.
6. What are the common oxidation states of noble metals?
Noble metals exhibit variable oxidation states, commonly positive oxidation numbers in compounds.
- Gold (Au): +1 and +3
- Silver (Ag): +1
- Platinum (Pt): +2 and +4
- Palladium (Pd): +2
7. How are noble metals extracted from their ores?
Noble metals are extracted using processes such as cyanide leaching, smelting, and electrorefining.
- Gold extraction often uses cyanidation: 4Au(s) + 8CN-(aq) + O2(g) + 2H2O(l) → 4[Au(CN)2]-(aq) + 4OH-(aq).
- The dissolved complex is later reduced to obtain pure gold.
- Platinum group metals are usually recovered as by-products of nickel and copper refining.
8. Why are noble metals used as catalysts?
Noble metals are used as catalysts because they provide active surfaces for adsorption while remaining chemically stable.
- Platinum (Pt) and palladium (Pd) are widely used in catalytic converters.
- In automobile converters, Pt and Pd help convert harmful gases: 2CO(g) + O2(g) → 2CO2(g).
- Their resistance to oxidation allows repeated catalytic cycles without degradation.
9. Are noble metals good conductors of electricity?
Yes, noble metals are excellent electrical conductors due to their delocalized metallic bonding and free-moving electrons.
- Silver (Ag) has the highest electrical conductivity of all metals.
- Gold (Au) is widely used in electronics because it resists corrosion.
- Their stability ensures long-term performance in circuits and connectors.
10. Where are noble metals located in the periodic table?
Noble metals are primarily located in the d-block (transition metals) of the periodic table, mainly in Groups 8–11.
- Gold (Au) and silver (Ag) are in Group 11.
- Platinum group metals occupy parts of Groups 8, 9, and 10.
- They are positioned toward the right side of the transition series, reflecting higher electronegativity and lower reactivity compared to other metals.
