Noble Metals: Definition, Properties, Use, and Types
Noble metals are a class of metallic elements known for their exceptional resistance to corrosion, oxidation, and chemical reactivity under most environmental conditions. They are sometimes referred to as “precious metals” due to their rarity and economic value, or as “coinage metals” in historical contexts, although these terms are not strictly synonymous. The noble metals represent a narrow group of metallic elements characterized by low chemical reactivity, especially with oxygen, acids, and moisture. In their broadest classification, noble metals include elements from Groups 8 to 11 of the periodic table, particularly ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), osmium (Os), iridium (Ir), platinum (Pt), and gold (Au). However, the exact list can vary depending on whether the focus is on chemical behavior, economic value, or historical usage.
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 explores the definition, chemical and physical properties, practical applications, and types of noble metals, highlighting their continued importance in both advanced technologies and traditional uses.
What Is a Noble Metal?
A noble metal is a metallic element that exhibits exceptional resistance to oxidation, corrosion, and other forms of chemical degradation. 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, electrical contacts, catalysis, and medical devices.
Their resistance to oxidation and ability to maintain luster over time further enhance their desirability.
A key example is gold (Au), one of the few elements naturally found in its metallic state in the Earth's crust, rather than as compounds. It does not readily form oxides or salts, and under normal environmental conditions, it maintains its characteristic luster and structural integrity for centuries without significant degradation.
What Are the Different Properties of Noble Metals?
Noble metals possess a unique combination of physical and chemical properties that distinguish them from most metallic elements. These characteristics account for their broad utility in industrial, electronic, medical, and decorative applications. Below are some of the most important properties of noble metals:
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 in 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 typically exhibit exceptional electrical conductivity, a property stemming from their highly mobile electrons in the outermost electron orbit. This configuration allows for the 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 are known for their characteristic metallic luster, a result of their ability to uniformly reflect visible light. This reflectivity is due to the interaction of light with the free electron cloud at the metal surface, leading to strong specular reflection and a bright, mirror-like appearance. This optical property enhances their desirability for ornamental, architectural, and aesthetic applications, especially in the jewelry, watchmaking, and art industries. The fact that this luster remains stable over time—without tarnishing or discoloration—further sets noble metals apart from more reactive alternatives.
What Is the Use of Noble Metals?
Noble metals are remarkably ubiquitous in applications across various industries due to their unique properties, 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 employ 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’ ability 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:
- Jewelry
- Electronics
- Electrical
- Chemical Processing
- Medical
- Finance
- Petroleum
- 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 are 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. However, extensive experience in prospecting and geology has developed a better understanding of the potential locations and characteristics of higher concentration regions. 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 (Au)
Gold is considered the archetype of noble metals due to its extraordinary resistance to oxidation and corrosion, as well as its unique yellow luster. It is chemically inert under most conditions, making it stable in air and moisture. Widely coveted for its intrinsic value and aesthetic appeal, gold is widely used 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 (Ag)
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 (Pd)
Palladium is a silvery-white noble metal noted for its excellent catalytic efficiency 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 (Ir)
Iridium is one of the densest and most corrosion-resistant elements, with a melting point exceeding 2,400°C. It retains its chemical stability even at elevated temperatures, making it ideal for use in spark plugs, electrical contacts, crucibles, and high-performance alloys. Due to its extreme durability, iridium is also used in scientific instrumentation, standard mass measurements, and deep-sea or space-exploration components. While rare and costly, iridium is valuable in highly specialized engineering and scientific applications, particularly in analytical chemistry and in manufacturing standard weights and measures that don’t change by corrosion.
5. Rhodium (Rh)
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 (Os)
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 partly 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 (Pt)
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:
- 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.
- Consider properties such as melting point, density, and mechanical strength based on the application's more extreme demands.
- 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, because of large price fluctuations.
- Determine if specific properties like electrical conductivity, thermal stability, or catalytic activity are necessitated by the rigors of the application.
- 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 allow 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. Palladium is similarly utilized in multilayer ceramic capacitors (MLCCs), sensor components, and specialized connectors, valued for its oxidation resistance and catalytic properties.
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:
- Exhibit exceptional resistance to oxidative and other forms of corrosion.
- Are universally excellent conductors of electricity. This allows for low-voltage drop transmission of the most delicate and tenuous electrical signals and currents.
- Platinum, palladium, and rhodium, in particular, possess powerful catalytic properties. This enables them to facilitate chemical reactions in industrial processes.
- Are generally considered biocompatible, making them suitable for medical implants, surgical tools, and treatments.
- 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.
Common FAQs About Noble Metals
How Do Corrosion-Resistant Noble Metals Differ From Rare Earth Metals Chemically?
Corrosion-resistant noble metals differ chemically from rare earth metals 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 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:
- 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.
- 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 often used in applications requiring high chemical stability, such 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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