Noble gases comprise six chemically inert elements: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). These gases occupy Group VIII of the periodic table and are characterized by their full valence electron shells, which confer exceptional stability and minimal reactivity.
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The unreactive nature of noble gases stems from their complete valence electron shells. For instance, helium, with an electronic configuration of $1s^2$, has a full first shell, while neon has $2s^2 2p^6$, completing its second shell. This full complement of electrons means noble gases do not readily gain or lose electrons, making them chemically inert under standard conditions.
Noble gases exist as monatomic species, meaning each gas consists of single atoms rather than molecules. This monatomic characteristic is a direct consequence of their stable electronic configurations, which do not favor the formation of bonds with other atoms. For example, argon exists as individual Ar atoms, not forming Ar₂ molecules.
Noble gases share several physical properties, including low boiling and melting points, which decrease with increasing atomic number. Helium, the lightest noble gas, has the lowest boiling point at −268.93°C, while radon, the heaviest, has a boiling point of −61.7°C. Their gaseous state at room temperature and low density make them ideal for applications like balloons and airships.
Noble gases are relatively rare in Earth's atmosphere, constituting about 0.93% by volume. They are primarily obtained through the fractional distillation of liquid air. Helium is extracted from natural gas deposits, while other noble gases like neon, argon, and krypton are separated from atmospheric air components due to their distinct boiling points.
The chemical inertness of noble gases means they seldom participate in chemical reactions. However, heavier noble gases like xenon and krypton can form compounds under specific conditions. For example, xenon can react with fluorine to form xenon hexafluoroplatinate (XePtF₆), demonstrating that noble gases can exhibit reactivity when subjected to extreme conditions.
Noble gases have a wide range of applications owing to their unique properties:
Unlike noble gases, many elements form diatomic molecules. For example, oxygen (O₂) and nitrogen (N₂) exist as diatomic gases under standard conditions. The monatomic nature of noble gases is a direct consequence of their filled valence shells, negating the need for bond formation to achieve stability.
Reactivity in the periodic table generally decreases across a period from left to right. Noble gases, located at the far right, exhibit the least reactivity. This trend highlights the influence of electron configuration on chemical behavior, with noble gases representing the pinnacle of inertness.
Noble gases possess high ionization energies and low electronegativities. High ionization energy indicates the difficulty in removing an electron, while low electronegativity reflects the minimal tendency to attract electrons. These properties contribute to the overall unreactive nature of noble gases.
Noble gases have several isotopes, some of which are stable while others are radioactive. For example, helium has two stable isotopes: helium-3 and helium-4. Radon, however, has no stable isotopes and is highly radioactive, decaying into other elements over time. The study of isotopes aids in understanding nuclear properties and applications in fields like medicine and archaeology.
While noble gases are generally non-toxic and inert, some, like radon, pose significant health risks due to their radioactivity. Proper handling and mitigation strategies are essential to ensure safety in environments where radioactive noble gases are present. Additionally, the use of noble gases in industrial applications must consider environmental implications, such as greenhouse gas potential and resource sustainability.
Noble gases are also integral to astrophysical studies. Their presence in stellar atmospheres helps astronomers determine elemental compositions and physical conditions of stars. Moreover, the detection of noble gases in interstellar mediums contributes to our understanding of cosmic processes and the evolution of galaxies.
The discovery of noble gases dates back to the late 19th century, with helium being the first identified in the solar spectrum before its terrestrial presence was confirmed. Subsequent discoveries of neon, argon, krypton, and xenon expanded the group, each unveiling unique properties and applications. Ongoing research into noble gases continues to reveal new compounds and potential uses in advanced technologies.
The commercial production of noble gases involves complex purification processes, primarily through fractional distillation of liquefied air. The economic viability of noble gases depends on their rarity and demand across various industries. Helium, for example, is a finite resource with critical applications, leading to concerns over its conservation and sustainable use.
Advancements in noble gas research promise novel applications and improved extraction methods. Innovations such as high-efficiency lighting, enhanced medical imaging techniques, and breakthroughs in cryogenics are on the horizon. Additionally, the exploration of noble gas compounds could lead to new materials with unique properties, expanding the horizons of chemistry and materials science.
At the molecular level, the inertness of noble gases is fundamentally attributed to their electronic configurations. The Aufbau principle explains how electrons fill atomic orbitals, resulting in filled valence shells for noble gases. For instance, neon has a complete $2s^2 2p^6$ configuration, achieving a stable, low-energy state. Quantum chemistry further elucidates that the lack of available low-energy vacant orbitals in noble gases prevents the formation of stable compounds, reinforcing their unreactive nature.
The quantum mechanical model offers a deeper understanding of noble gas stability. According to the Schrödinger equation, atoms possess discrete energy levels. Noble gases have their electrons arranged in such a way that all orbitals are fully occupied, minimizing potential energy and maximizing stability. This electron configuration leads to filled s and p orbitals, making it energetically unfavorable for noble gases to engage in bonding interactions.
While noble gases are generally unreactive, under high-energy conditions, such as ionization or excitation by electrical discharge, they can form ions or excited states. For example, in gas discharge tubes, applying sufficient energy can remove an electron from a noble gas atom, forming a noble gas cation: $$\text{Ne} \rightarrow \text{Ne}^+ + e^-$$ These ions can then interact to form transient compounds or emit characteristic spectral lines upon returning to their ground state, which is the basis for technologies like neon lighting.
The synthesis of noble gas compounds, though challenging, has been achieved primarily with heavier noble gases like xenon and krypton. These compounds typically require the presence of highly electronegative elements or strong oxidizing agents. A notable example is xenon hexafluoroplatinate ($\text{XePtF}_6$), synthesized through the reaction: $$\text{Xe} + 3\text{PtF}_4 \rightarrow \text{XePtF}_6$$ The creation of such compounds expands our understanding of chemical bonding and challenges the notion of absolute inertness in noble gases.
Spectroscopic techniques, including emission and absorption spectroscopy, are vital for studying noble gases. When excited, noble gas atoms emit light at specific wavelengths unique to each element, allowing for their identification and analysis. For example, helium emits light at wavelengths corresponding to transitions in its electron energy levels, which are pivotal in astrophysical observations and plasma diagnostics.
Under extreme temperatures and pressures, noble gases can transition into supercritical fluids, possessing properties of both liquids and gases. Supercritical xenon, for instance, exhibits high solvating power and is used in supercritical fluid extraction processes. The study of supercritical noble gases contributes to advancements in materials science and chemical engineering.
Noble gases are invaluable in low-temperature physics research. Helium-4 remains liquid down to absolute zero under standard pressure, making it essential for achieving cryogenic temperatures. Liquid helium is used to cool superconducting magnets, enabling technologies like MRI machines and particle accelerators. The unique thermodynamic properties of noble gases facilitate the exploration of quantum phenomena and superconductivity.
Noble gas isotopes serve as geochemical tracers, providing insights into geological processes and the history of Earth's atmosphere. For example, helium isotopes can indicate mantle-derived gases and contribute to our understanding of volcanic activity. Similarly, neon and argon isotopic ratios help in dating geological formations and studying planetary differentiation.
In plasma physics, noble gases are commonly used due to their inertness and ability to sustain stable plasma states. Argon plasma, for instance, is utilized in plasma cutting and surface treatment applications. Understanding the behavior of noble gases in plasma states is essential for advancements in fusion research and the development of plasma-based technologies.
Emerging research explores the role of noble gases in quantum computing. Helium-3, with its unique nuclear spin properties, is investigated for use in quantum bits (qubits) due to its low magnetic interaction and long coherence times. The stable and unreactive nature of noble gases makes them ideal candidates for creating controlled quantum environments.
The extraction and utilization of noble gases raise environmental considerations. For example, helium extraction from natural gas reservoirs must balance industrial demand with conservation efforts, as helium is a non-renewable resource on Earth. Efficient extraction technologies and recycling methods are vital to mitigate environmental impact and ensure sustainable use of noble gases.
Noble gases intersect with various scientific disciplines, including medicine. Helium-neon lasers are used in ophthalmology for corrective eye surgeries, while xenon-based anesthetics offer advantages in patient care due to their rapid induction and recovery times. The interdisciplinary applications of noble gases highlight their versatility beyond traditional chemistry.
Innovative technologies harnessing noble gases are on the rise. For instance, xenon is being explored as a propellant in ion thrusters for spacecraft, leveraging its high atomic mass for efficient propulsion. Additionally, krypton is utilized in advanced lighting systems with enhanced energy efficiency and extended lifespan. These emerging applications underscore the ongoing relevance of noble gases in technological advancement.
While noble gases themselves do not directly contribute to climate change, their applications intersect with environmental sustainability. For example, argon used in inert gas welding helps reduce atmospheric emissions during metal fabrication. Furthermore, helium conservation efforts are linked to sustainable practices in scientific research and medical industries, indirectly influencing climate change mitigation strategies.
Computational chemistry plays a pivotal role in predicting and modeling noble gas compounds. Advanced simulations using quantum mechanical methods allow scientists to explore potential bonding scenarios and stability of hypothetical noble gas compounds. These computational studies facilitate the design of experiments, saving time and resources in the pursuit of novel chemical species.
The future of noble gas research lies in uncovering new compounds, enhancing extraction methods, and expanding applications in emerging technologies. Collaborative interdisciplinary studies will likely drive innovations, while sustainable practices will ensure the responsible use of noble gas resources. Continued exploration promises to reveal deeper insights into atomic behavior and unlock new scientific frontiers.
Understanding noble gases often involves grappling with complex chemical principles and experimental techniques. For example, predicting the formation of noble gas compounds requires sophisticated theoretical models that account for weak interactions and high-energy conditions. Additionally, isolating and characterizing transient noble gas species demands advanced instrumentation and meticulous experimental design, presenting significant challenges to chemists.
Mathematical models play an essential role in describing noble gas behavior. Quantum mechanical equations, such as the Schrödinger equation, are employed to predict electron configurations and energy states. Additionally, thermodynamic equations help quantify properties like ionization energy and excitation states. Mastery of these mathematical models is crucial for advancing theoretical and applied chemistry involving noble gases.
Experimental methodologies are fundamental to exploring noble gas properties. Techniques like mass spectrometry, gas chromatography, and spectroscopy are routinely used to analyze noble gas samples and their interactions. High-precision measurements enable the detection of subtle changes in noble gas behavior under varying conditions, facilitating breakthroughs in understanding their chemistry.
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The extraction and use of noble gases raise ethical questions related to resource allocation and environmental stewardship. Helium scarcity, for instance, prompts debates on its prioritization for critical applications versus commercial uses. Ethical considerations also extend to the handling of radioactive noble gases like radon, ensuring public safety and minimizing environmental contamination.
A noble metal is ordinarily regarded as a metallic element that is generally resistant to corrosion and is usually found in nature in its raw form. Gold, platinum, and the other platinum group metals (ruthenium, rhodium, palladium, osmium, iridium) are most often so classified. Silver, copper, and mercury are sometimes included as noble metals, but each of these usually occurs in nature combined with sulfur.
In more specialized fields of study and applications the number of elements counted as noble metals can be smaller or larger. It is sometimes used for the three metals copper, silver, and gold which have filled d-bands, while it is often used mainly for silver and gold when discussing surface-enhanced Raman spectroscopy involving metal nanoparticles. It is sometimes applied more broadly to any metallic or semimetallic element that does not react with a weak acid and give off hydrogen gas in the process. This broader set includes copper, mercury, technetium, rhenium, arsenic, antimony, bismuth, polonium, gold, the six platinum group metals, and silver.
Many of the noble metals are used in alloys for jewelry or coinage. In dentistry, silver is not always considered a noble metal because it is subject to corrosion when present in the mouth. All the metals are important heterogeneous catalysts.
While lists of noble metals can differ, they tend to cluster around gold and the six platinum group metals: ruthenium, rhodium, palladium, osmium, iridium, and platinum.
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.[5]
The term noble metal can be traced back to at least the late 14th century[6] and has slightly different meanings in different fields of study and application.
Prior to Mendeleev's publication in of the first (eventually) widely accepted periodic table, Odling published a table in , in which the "noble metals" rhodium, ruthenium, palladium; and platinum, iridium, and osmium were grouped together,[7] and adjacent to silver and gold.
The noble metals are siderophiles (iron-lovers). They tend to sink into the Earth's core because they dissolve readily in iron either as solid solutions or in the molten state. Most siderophile elements have practically no affinity whatsoever for oxygen: indeed, oxides of gold are thermodynamically unstable with respect to the elements.
Copper, silver, gold, and the six platinum group metals are the only native metals that occur naturally in relatively large amounts.[8]
Noble metals tend to be resistant to oxidation and other forms of corrosion, and this corrosion resistance is often considered to be a defining characteristic. Some exceptions are described below.
Copper is dissolved by nitric acid and aqueous potassium cyanide.
Ruthenium can be dissolved in aqua regia, a highly concentrated mixture of hydrochloric acid and nitric acid, only when in the presence of oxygen, while rhodium must be in a fine pulverized form. Palladium and silver are soluble in nitric acid, while silver's solubility in aqua regia is limited by the formation of silver chloride precipitate.[9]
Rhenium reacts with oxidizing acids, and hydrogen peroxide, and is said to be tarnished by moist air. Osmium and iridium are chemically inert in ambient conditions.[10] Platinum and gold can be dissolved in aqua regia.[11] Mercury reacts with oxidising acids.[10]
In , US researchers discovered that an organic "aqua regia" in the form of a mixture of thionyl chloride SOCl2 and the organic solvent pyridine C5H5N achieved "high dissolution rates of noble metals under mild conditions, with the added benefit of being tunable to a specific metal" for example, gold but not palladium or platinum.[12]
However, Gold can be dissolved in selenic acid (H2SeO4).
The noble elements gold and platinum also have a comparatively high electronegativity for a metallic element, thus allowing them to exist as single-metallic anions.
For example:
Cs + Au -> CsAu
(Caesium auride, a yellow crystalline salt with the Au−
ion).[citation needed] Platinum also exhibits similar properties with
BaPt, BaPt2, Cs2Pt (Barium and Caesium Platinides, which are reddish salts).[13][14]
The expression noble metal is sometimes confined to copper, silver, and gold since their full d-subshells can contribute to their noble character.[15] There are also known to be significant contributions from how readily there is overlap of the d-electron states with the orbitals of other elements, particularly for gold.[16] Relativistic contributions are also important,[17] playing a role in the catalytic properties of gold.[18]
The elements to the left of gold and silver have incompletely filled d-bands, which is believed to play a role in their catalytic properties. A common explanation is the d-band filling model of Hammer and Jens Nørskov,[19][20] where the total d-bands are considered, not just the unoccupied states.
The low-energy plasmon properties are also of some importance, particularly those of silver and gold nanoparticles for surface-enhanced Raman spectroscopy, localized surface plasmons and other plasmonic properties.[21][22]
Standard reduction potentials in aqueous solution are also a useful way of predicting the non-aqueous chemistry of the metals involved. Thus, metals with high negative potentials, such as sodium, or potassium, will ignite in air, forming the respective oxides. These fires cannot be extinguished with water, which also react with the metals involved to give hydrogen, which is itself explosive. Noble metals, in contrast, are disinclined to react with oxygen and, for that reason (as well as their scarcity) have been valued for millennia, and used in jewellery and coins.[23]
Electrochemical properties of some metals and metalloids Element Z G P Reaction SRP(V) EN EA Gold ✣ 79 11 6 Au3+The adjacent table lists standard reduction potential in volts;[24] electronegativity (revised Pauling); and electron affinity values (kJ/mol), for some metals and metalloids.
The simplified entries in the reaction column can be read in detail from the Pourbaix diagrams of the considered element in water. Noble metals have large positive potentials;[25] elements not in this table have a negative standard potential or are not metals.
Electronegativity is included since it is reckoned to be, "a major driver of metal nobleness and reactivity".[3]
The black tarnish commonly seen on silver arises from its sensitivity to sulphur containing gases such as hydrogen sulfide:
Rayner-Canham[4] contends that, "silver is so much more chemically-reactive and has such a different chemistry, that it should not be considered as a 'noble metal'." In dentistry, silver is not regarded as a noble metal due to its tendency to corrode in the oral environment.[26]
The relevance of the entry for water is addressed by Li et al.[27] in the context of galvanic corrosion. Such a process will only occur when:
The superheavy elements from hassium (element 108) to livermorium (116) inclusive are expected to be "partially very noble metals"; chemical investigations of hassium has established that it behaves like its lighter congener osmium, and preliminary investigations of nihonium and flerovium have suggested but not definitively established noble behavior.[28] Copernicium's behaviour seems to partly resemble both its lighter congener mercury and the noble gas radon.[29] Moscovium has been also investigated to behave similarly to its lighter congener bismuth.[30]
As long ago as , Hiorns observed as follows:
Smith, writing in , continued the theme:
Such nobility is mainly associated with the relatively high electronegativity values of the noble metals, resulting in only weakly polar covalent bonding with oxygen.[3] The table lists the melting points of the oxides of the noble metals, and for some of those of the non-noble metals, for the elements in their most stable oxidation states.
All the noble metals can act as catalysts. For example, platinum is used in catalytic converters, devices which convert toxic gases produced in car engines, such as the oxides of nitrogen, into non-polluting substances.[citation needed]
Gold has many industrial applications; it is used as a catalyst in hydrogenation and the water gas shift reaction.[34]
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