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Noble Metals: Definition, Properties, Use, and Types

Xomety X
By Team Xometry
April 26, 2024
 15 min read
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Noble metals are sometimes described as the metallic elements that oxidize least, or as ‘coin metals’ or as precious metals. The noble metals represent a narrow group of metallic elements of exceptional resistance to corrosion and oxidation. This category, at its widest, encompasses metals located in Groups 8-11 of the periodic table including: gold (Au), silver (Ag), palladium (Pd),  iridium (Ir), and platinum (Pt) among others.

Their unique properties include high ductility, high electrical and thermal conductivity, and varying degrees of chemical inertness, which render them indispensable in industries requiring these unusual properties.

This article will discuss what noble metals are, their properties, uses, and types.

What Is a Noble Metal?

Noble metals can be distinguished by their exceptional resistance to corrosion/oxidation and other forms of chemical attack. This property arises from their stable crystalline and metallic-bonded formulations and satisfied outer valencies. Unlike most others, noble metals do not readily react with air or moisture, making them highly inert and chemically durable. Additionally, noble metals typically offer high ductility/malleability and conductivity, contributing to their exploitation in various applications, including: jewelry, electronics, and catalysis.

Their resistance to oxidation and ability to maintain their luster over time further enhance their desirability. Gold is one of the few metallic elements found widely in the environment in its metallic form (though generally at very low concentrations). It produces no natural salts or reaction byproducts. 

What Are the Different Properties of Noble Metals?

The different properties of noble metals are briefly discussed below:

1. High Corrosion Resistance

Noble metals benefit from exceptional corrosion resistance and their (more or less) inert nature. Gold, silver, platinum, rhodium, and palladium experience minimal reactivity with oxygen, moisture, and corrosive substances, diminishing tarnishing and mass loss/degradation over time. This is due to their stable crystalline structural configurations and minimal (or entirely satisfied) available valencies. 

This inherent resistance to corrosion makes noble metals highly appreciated for applications in harsh environments, such as marine and chemical industries, in which protection against oxidation and other chemical reactions is essential. Their function as jewelry can also be considered as a moderately harsh environment, given the variable acidity of human sweat. This corrosion resistance greatly contributes to their longevity and aesthetic appeal in jewelry and decorative items.

2. Excellent Electrical Conductivity

Noble metals exhibit typically exceptional electrical conductivity, a property stemming from their highly mobile electrons in the outermost electron orbit. This configuration allows for efficient movement of electric charge between atoms with minimal resistance, making noble metals ideal conductors. Copper and, where necessary, gold and silver, among others, are extensively used in electrical wiring, connectors, and circuitry due to their high conductivity. The superior electrical conductivity of noble metals contributes to their widespread use in various technological and industrial applications, such as the first-stage connections between semiconductor devices and their housings.

3. Distinctive Luster

Noble metals possess a characteristic luster that sets them apart from other metals. This luster is a result of their ability to reflect light uniformly across their surface, creating a brilliant shine. This distinctive property enhances the aesthetic appeal of noble metals, making them highly desirable for decorative and ornamental purposes.

What Is the Use of Noble Metals?

Noble metals are remarkably ubiquitous in applications across various industries due to their unique properties and despite their high value. Jewelry and ornamental items are made from noble metals like: gold, silver, and platinum—materials highly valued for their enduring luster due to limited oxidation. Electronics employs noble metals extensively in electrical contacts and connectors due to their excellent conductivity and resistance to corrosion, ensuring reliable performance in electronic devices. Catalysis using platinum and palladium is widely exploited in accelerating organic chemical reactions due to the metals’ facility as intermediaries to facilitate and speed reactions without being consumed. Medical implants exploit noble metals because of their biocompatibility, making them optimal for patient tissue implants such as: pacemakers, stents, and dental prostheses. 

What Industry Uses Noble Metals?

Some industries that use noble metals are:

  1. Jewelry
  2. Electronics
  3. Electrical
  4. Chemical Processing
  5. Medical
  6. Finance
  7. Petroleum
  8. Marine

How Do Noble Metals Improve Healthcare, Particularly in Medical Devices and Cancer Treatment Development?

Noble metals hold a pivotal role in healthcare, especially in medical devices and cancer-treatment development, due to their chemical and (to some extent) catalytic properties. In medical devices, noble metals like: platinum, gold, and silver are highly biocompatible and tissue-accepted, making them suitable for implants such as: pacemakers, stents, and dental prostheses. Additionally, their inertness and resistance to corrosion ensure long-term compatibility within the body. In cancer treatment, noble metals such as platinum-based drugs are used in chemotherapy to target and destroy cancer cells. These drugs, like cisplatin, carboplatin, and oxaliplatin, inhibit cell division and induce apoptosis (normal and systemic controlled cell death to reduce faulty development) in cancer cells, offering effective therapeutic options for patients with various types of cancer.

Are Rare Earth Metals a Subset of Lanthanides?

No, although the two groups are closely parallel. Lanthanides are a subset of the rare earth metals, a group that also includes scandium and yttrium which are not lanthanides. They have closely related properties and are commonly found in multi-metal ores with several metals represented. No members of either the lanthanides or the rare earth metals are members of the noble metals classification.

Are Noble Metals Hard To Source?

Yes. Noble metals are generally abundant in the Earth's crust, but highly distributed such that commercial extraction is typically impractical. More concentrated sources are rarely easy to find due to factors such as geological distribution, and the practicality of extraction is highly dependent on market demand. Long experience in prospecting and geology has developed a better understanding of the potential locations and characteristics of higher concentration regions, however. While the availability of noble metals is highly variable, their extraction and refining processes can be complex and expensive adding greatly to their scarcity value and disruptive/harmful environmental impact. Additionally, factors such as geopolitical considerations and environmental regulations can also affect their sourcing, as extraction can be both highly toxic and labor-intensive.

What Are the Different Types of Noble Metals?

The different types of noble metals are listed below:

1. Gold

Gold is the “original” noble metal of considerable rarity and enduring attraction due to its direct correlation with wealth. It is unique in having a distinctive yellow luster and remarkable resistance to corrosion and tarnishing. Widely coveted for its intrinsic value and aesthetic appeal, gold finds diverse applications in: jewelry, currency, electronics, and dentistry. Additionally, its extraordinarily high conductivity and biocompatibility make it invaluable in tissue-implanted medical devices and treatments. As a visible and portable proxy for wealth and prosperity throughout history, gold continues to captivate and inspire both artisans and investors alike.

2. Silver

Silver possesses a brilliant white luster when polished and exceptional electrical and thermal conductivity. Appreciated for its aesthetic appeal and versatility across numerous functional uses, silver is widely used in jewelry, tableware, and decorative arts for its high quality and moderate price (for a noble metal). Its highly effective antimicrobial properties make it efficacious in medical devices and wound dressings, often in colloidal form (finely divided pure metal in a suspension). Additionally, silver nitrate is crucial in traditional “film” photography, mirrors (in amalgam with mercury as the reflective surface), and electronics due to its reflective and conductive nature. With a rich human and technology history spanning millennia, silver remains prized for its aesthetic appeal, practical utility, and enduring cultural significance.

3. Palladium

Palladium boasts remarkable catalytic properties and corrosion resistance. Widely used in automotive catalytic converters, palladium efficiently catalyzes the oxidation of otherwise harmful hydrocarbon and carbon-monoxide emissions into less toxic post-combustion products. Its applications extend to electronics, dentistry, and jewelry, reflecting its versatility and importance in a surprising range of industries and functions. As a precious metal, palladium also serves as an investment asset and store of value.

4. Iridium

Iridium is a particularly rare and dense noble metal, with exceptional corrosion resistance, even at elevated temperatures, making it invaluable in a considerable number of specialized applications. It is commonly utilized in spark plugs, electrical contacts, and precious-metals crucibles due to its arc-resilient durability and high melting point. Iridium's scarcity and unique properties also encourage its use in fine jewelry such as high-value watches, but only whenever material price is of no concern. Additionally, it plays a role in scientific research, particularly in analytical chemistry and in the manufacture of standard weights and measures that don’t change by corrosion.

5. Rhodium

Rhodium is another rare and high-luster noble metal, celebrated for its equally remarkable corrosion resistance. It is prominently employed as a plating material for jewelry, ensuring a bright and reflective finish while using minimal quantities, adding only moderate cost with a high appearance value. Rhodium also serves well as a reaction catalyst in catalytic converters for automobiles, in which it helps combust hydrocarbon and carbon-monoxide emissions and has a long functional life due in part to its extreme durability. It is utilized in highly critical electrical contacts and as a catalyst in various chemical reactions, presenting as versatile across multiple industries, despite its extreme cost. Due to its scarcity, rhodium commands a high price in the market.

6. Osmium

Osmium is among the highest-density naturally occurring elements and is another member of the platinum group. Handled with care due to its high toxicity, exploited for its hardness and extremely high melting point, it is primarily utilized in alloying to enhance the durability of electrical contacts, fountain-pen nibs, and other mechanical components as a minor constituent with significant property influence. This limited usage is in part due to its rarity/cost and because large benefits accrue in alloys with a relatively low osmium content.

Additionally, its compounds are employed in staining tissues for microscopy and as catalysts in organic synthesis reactions in which OsO4 serves in oxidizing alkenes to form vicinal diols, a group of specialist glycols. Despite its practical applications, osmium's scarcity and toxicity pose challenges, limiting its widespread use compared to other noble metals.

7. Platinum

Platinum is valuable due to its rarity, aesthetics, and durability. It finds extensive use in various industrial and commercial applications as a catalyst in chemical reactions, notably in catalytic converters to reduce unreacted hydrocarbon emissions from vehicle exhausts. Additionally, platinum's resistance to corrosion and tarnishing makes it a preferred choice for jewelry and small, high-value decorative items. Its conductivity properties make it valuable in electrical-contact applications, while its biocompatibility renders it ideal for medical implants. Platinum is also utilized in various laboratory equipment and fuel cells for clean electricity production from chemical source materials.

How To Choose Which Type of Noble Metal To Use

Choosing the right type of noble metal relies entirely on the specific requirements of the application. Factors to consider include:

  1. Assess the aggressive nature of the chemical environment the metal will be exposed to and choose a metal with appropriate facility in resistance to corrosion and oxidation, balancing the material cost against durability.
  2. Consider properties such as: melting point, density, and mechanical strength based on the application's more extreme demands.
  3. Evaluate the cost-effectiveness of each noble metal option, considering availability, purity, efficacy in the task, and market-price fluctuations. More unusual metals can pose serious sourcing risks in long-term production and because of large price fluctuations.
  4. Determine if specific properties like: electrical conductivity, thermal stability, or catalytic activity are necessitated by the rigors of the application.
  5. Ensure compliance with regulatory standards or industry specifications that may apply to the choice of noble metal. These might be military, medical, environmental, etc.

What Type of Noble Earth Metal Is Used in the Electronics Industry?

In the electronics industry, silver is commonly used due to its excellent electrical conductivity and relatively low cost. Silver is particularly useful in components like: connectors, switches, and switch contacts. Gold is also utilized in electronics for its corrosion resistance and reliability in low-voltage applications in which oxidation could disrupt conductivity. Electroplating and physical vapor deposition allows very thin coatings that can be hard wearing and use very little metal.

Platinum is occasionally employed in high-temperature and corrosive environments in applications like sensors. Meanwhile, palladium is also used in electronic-sensor components due to its catalytic properties and resistance to oxidation.

What Type of Noble Metal Is Used in Petrochemical Applications?

Platinum, rhodium, and palladium are occasionally used in petrochemical applications as catalysts, due to their ability to facilitate and optimize various beneficial chemical reactions while withstanding harsh operating conditions unchanged.

These catalysts are utilized in processes like catalytic reforming. They accelerate the conversion of hydrocarbons into higher-octane gasoline components. Additionally, platinum, palladium, and rhodium are used in catalytic converters within vehicles to reduce hydrocarbon emissions from exhaust gases. These noble metal catalysts play crucial roles in enhancing the efficiency and environmental friendliness of petrochemical processes.

What Is the Advantage of Using Noble Metals?

The advantages of using noble metals include:

  1. Exhibit exceptional resistance to oxidative and other forms of corrosion.
  2. Are universally excellent conductors of electricity. This allows for low-voltage drop transmission of the most delicate and tenuous electrical signals and currents.
  3. Platinum, palladium, and rhodium, in particular, possess powerful catalytic properties. This enables them to facilitate chemical reactions in industrial processes.
  4. Are generally considered biocompatible, making them suitable for medical implants, surgical tools, and treatments.
  5. Noble metals, particularly gold and platinum, have a distinctive luster and appearance that are highly valued in jewelry and decorative applications.

What Is the Disadvantage of Using Noble Metals?

The primary disadvantage of using noble metals is their extremely high cost compared to essentially all other materials. Due to their rarity and desirable properties, gold, platinum, and palladium are extremely expensive to acquire and require very conservative usage/processing. This renders products and applications exploiting noble metals very costly to manufacture, limiting their use to applications with low to zero budget sensitivity. Additionally, the high value and portability of noble metals make them attractive targets for theft and illicit trade, requiring stringent controls and security measures in their handling and storage. 

Despite these drawbacks, the unique properties of noble metals often necessitate their use in specific applications in which their benefits are imperative and compensate for the high costs involved.

How Do Corrosion-Resistant Noble Metals Differ From Rare Earth Metals Chemically?

Corrosion-resistant noble metals differ from rare earth metals chemically in various intrinsic ways. Noble metals are characterized by their inertness and resistance to corrosion, oxidation, and chemical reactions, which make them highly stable in virtually all environments. Rare earth metals, although also exhibiting moderate chemical stability, are more reactive and prone to oxidation and corrosion. Rare earth metals tend to form compounds readily, often exhibiting multiple oxidation states and complex bonding behavior. Additionally, rare earth metals are typically found in the f-block of the periodic table, while noble metals are located in the d-block.

The applications for rare earth metals and noble metals show little overlap. Rare earth metals are being applied as key metallic or metal-salt components in glass, LED lighting, magnets, and lithium polymer batteries used in virtually every portable product. The one area of commonality between the two classes of metals is their use in catalysts, in which automotive catalytic converters can employ both noble and rare earth metals.

Are Precious Metals the Same as Noble Metals?

No, although precious metals and noble metals share some similarities in characteristics and properties, they cannot be analyzed as one group. Precious metals reference a specific group of metallic elements that are valued for their rarity and beauty, currency/wealth associations as well as a spectrum of industrial applications.

Noble metals are a subset of precious metals, consisting of metals that exhibit outstanding resistance to corrosion and oxidation in moist air. These metals include: gold, silver, platinum, palladium, rhodium, iridium, and osmium.

So, while all noble metals are precious metals, not all precious metals are necessarily noble metals.

What Is the Difference Between Precious Metals and Noble Metals?

The main difference between precious metals and noble metals lies in their definitions and characteristics, such as:

  1. Precious metals are metallic elements valued for their rarity, aesthetic appeal, currency/wealth association, and various industrial applications. This group includes: gold, silver, platinum, and palladium. Precious metals are often used in jewelry, currency, electronics, and a variety of high-value applications.
  2. Noble metals are a subset of the precious metals grouping, consisting of metals that possess exceptionally high resistance to corrosion and oxidation in an open atmosphere. Noble metals include: gold, silver, platinum, palladium, rhodium, iridium, and osmium. These metals are highly chemically inert and are often used in applications requiring high chemical stability, as catalysts and electrical contacts.

Although all noble metals are precious metals, not all precious metals can be defined as noble metals. Noble metals have some more-extreme chemical properties that make them additionally resistant to corrosion and oxidation.

Summary

This article presented noble metals, explained each of them, and discussed their various properties and applications. To learn more about noble metals, contact a Xometry representative.

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Xomety X
Team Xometry
This article was written by various Xometry contributors. Xometry is a leading resource on manufacturing with CNC machining, sheet metal fabrication, 3D printing, injection molding, urethane casting, and more.