Explanation: Jöns Jacob Berzelius discovered selenium in 1817 and named it after 'selene,' the Greek word for the moon, due to its perceived similarity to tellurium, which was named after the Earth.

Explanation: Selenium is relatively rare in the Earth's crust and seldom occurs in its pure elemental state or as distinct selenium ores. It is most commonly found as an impurity within metal sulfide ores, substituting for sulfur.

Explanation: The gray, hexagonal form of selenium is its most stable allotrope, characterized by a dense crystalline structure composed of helical polymeric chains.

Explanation: Gray selenium is insoluble in carbon disulfide and is notably resistant to oxidation by air and non-oxidizing acids, distinguishing it from other allotropes.

Explanation: Selenium has seven naturally occurring isotopes, five of which are stable (74Se, 76Se, 77Se, 78Se, 80Se). The isotope 82Se is a primordial radionuclide with a very long half-life, and 79Se is a fission product with a long half-life.

Explanation: The most stable synthetic isotope, 75Se, has a half-life of approximately 119.78 days, not hours. Other synthetic isotopes like 72Se have half-lives measured in days.

Explanation: Selenium is a chemical element with the atomic number 34 and is represented by the symbol Se.

Explanation: Selenium was discovered by the Swedish chemist Jöns Jacob Berzelius in 1817.

Explanation: Selenium is rarely found in its pure elemental form or as distinct ores. It is most commonly encountered as a trace impurity within metal sulfide ores, where it substitutes for sulfur atoms in the crystal lattice.

Explanation: The gray, hexagonal form of selenium is its most stable allotrope, characterized by a dense crystalline structure composed of helical polymeric chains.

Explanation: Selenium commonly exhibits oxidation states of -2, +4, and +6 in its compounds. The states -1 and +3 are less typical or observed under specific conditions, and +5 is not a common oxidation state for selenium.

Explanation: Selenium dioxide (SeO2) is formed through the direct combustion of elemental selenium in an oxygen atmosphere.

Explanation: Selenous acid (H2SeO3) is formed when selenium dioxide (SeO2) dissolves in water. Selenium trioxide (SeO3) is less stable and reacts differently.

Explanation: Selenium hexafluoride (SeF6) is generally considered more reactive and a greater pulmonary irritant compared to sulfur hexafluoride (SF6).

Explanation: Hydrogen selenide (H2Se) is a highly toxic and unstable gas, although it shares structural similarities with hydrogen sulfide (H2S).

Explanation: Tetraselenium tetranitride (Se4N4) is an explosive compound, analogous to tetrasulfur tetranitride, and is not used in fertilizers.

Explanation: Organoselenium compounds, including selenides (R2Se), diselenides (R2Se2), and selenols (RSeH), exhibit structural similarities to their organosulfur counterparts, reflecting selenium's position in the same group as sulfur in the periodic table.

Explanation: The common oxidation states for selenium in its compounds are -2, +4, and +6. The oxidation state +5 is not typically observed for selenium.

Explanation: Zinc selenide (ZnSe) is cited as an example of a metal selenide compound that functions as a semiconductor.

Explanation: Selenomethionine is an amino acid that incorporates selenium, making it a key example of an organoselenium compound found in biological systems.

Explanation: While selenium can be incorporated into certain alloys, such as brass, to enhance machinability, it is not primarily known as a structural metal itself. Its applications are more diverse, including glass production and semiconductor technology.

Explanation: Selenium is widely employed in the glass industry to neutralize the green tint caused by iron impurities, acting as a decolorizer, and also to impart a desirable red color to glass.

Explanation: Willoughby Smith's seminal 1873 discovery revealed that the electrical conductivity of selenium is significantly affected by light, a phenomenon known as photoconductivity, which paved the way for its use in light-sensing devices.

Explanation: Selenium rectifiers, developed in the early 20th century, were an important early application leveraging selenium's semiconductor properties for converting alternating current to direct current. They were eventually replaced by more efficient silicon-based devices.

Explanation: Selenium is not primarily extracted from crude oil refining. Its main industrial source is as a byproduct of copper electrolytic refining, where it is recovered from anode slimes.

Explanation: Elemental selenium is typically produced by processing copper refinery residues. This involves oxidizing the selenium-containing materials to form selenium dioxide, which is then further processed and reduced to elemental selenium.

Explanation: The largest commercial use of selenium is in the glass industry (approximately 50% of consumption) for decolorizing and coloring. While it is used in semiconductors, this is not its largest application.

Explanation: Selenium is incorporated into brass alloys, often alongside bismuth, to enhance machinability and provide an alternative to lead, particularly in applications related to potable water systems.

Explanation: Amorphous selenium thin films are key components in flat-panel X-ray detectors, functioning as photoconductors that convert X-ray photons directly into electrical signals, enabling high-resolution medical imaging.

Explanation: In X-ray crystallography, selenium is incorporated into molecules (often replacing sulfur) to aid in phasing techniques, such as MAD and SAD, which are essential for determining the three-dimensional structure of proteins and other macromolecules. It is not used as a radiation source.

Explanation: Selenium toning is a photographic process used to enhance the archival permanence and modify the tonal characteristics of black-and-white prints, often imparting a warmer tone.

Explanation: A primary challenge in enhancing the efficiency of selenium solar cells is overcoming a deficit in open-circuit voltage, rather than achieving a high one. Research focuses on improving carrier lifetime and reducing recombination.

Explanation: Key industrial applications of selenium include its use as a semiconductor material in photocells, light meters, and other electronic devices, as well as its role in glass manufacturing and pigment production.

Explanation: The discovery in 1873 that gray selenium exhibits photoconductivity—meaning its electrical conductivity changes with light intensity—was pivotal, leading to its application in light-sensing devices like photocells.

Explanation: The principal industrial source of selenium is as a byproduct recovered during the electrolytic refining of copper, where it accumulates in the anode slimes.

Explanation: Selenium is added to glass primarily to neutralize unwanted greenish tints caused by iron impurities (decolorizing) and to impart a distinct red color, widely used for decorative glass and signal lenses.

Explanation: In traditional black-and-white photography, selenium is employed in print toning processes to improve the longevity of prints and to subtly alter their tonal range, often yielding warmer hues.

Explanation: Selenium rectifiers were crucial early semiconductor devices used primarily for converting alternating current (AC) to direct current (DC) in electronic power supplies.

Explanation: A primary challenge in enhancing the efficiency of selenium solar cells is overcoming a deficit in open-circuit voltage. Research focuses on defect engineering to improve carrier lifetime and mitigate recombination losses.

Explanation: Amorphous selenium thin films function as photoconductors in flat-panel X-ray detectors, directly converting incident X-ray photons into electrical charge carriers, which are then read out to form an image.

Explanation: While selenium can be toxic in excessive amounts, trace quantities are essential for numerous biological functions, particularly as a component of antioxidant enzymes. Its toxicity is highly dependent on the form and dose.

Explanation: Selenium is an essential trace element for humans and many animals. It is a critical component of various enzymes, including antioxidant enzymes and those involved in thyroid hormone metabolism.

Explanation: Pure selenium deficiency is relatively rare globally. While it can exacerbate other health issues or contribute to conditions like Kashin-Beck disease under specific circumstances, it does not typically lead to severe neurological disorders in isolation without other contributing factors.

Explanation: A garlic odor on the breath is indeed a symptom of selenium toxicity (selenosis), but it is often accompanied by other significant symptoms such as hair loss, nail changes, and gastrointestinal distress, indicating a potentially serious condition rather than a mild one.

Explanation: Conversely, increased dietary selenium intake has been shown to help mitigate the effects of mercury toxicity, particularly at moderate exposure levels, by forming complexes with mercury and potentially supporting cellular defense mechanisms.

Explanation: Brazil nuts are renowned for their exceptionally high selenium content, often exceeding that found in common dietary sources like meats and cereals, though the exact amount can vary based on soil conditions.

Explanation: The Recommended Dietary Allowance (RDA) for selenium for adults in the United States is 55 micrograms (μg) per day, not 550 micrograms.

Explanation: Selenium is crucial for thyroid hormone metabolism, but not as a component of the hormones themselves. It acts as a cofactor for deiodinase enzymes, which are responsible for activating and deactivating thyroid hormones.

Explanation: Selenocysteine and selenomethionine are key organoselenium compounds found in biological systems, where they function analogously to their sulfur-containing counterparts (cysteine and methionine) within proteins.

Explanation: The identification of selenocysteine as a component of certain proteins was significant because it demonstrated that the UGA codon, typically recognized as a stop signal, can instead specify the incorporation of an amino acid during protein synthesis.

Explanation: Selenium and iodine have closely intertwined roles in thyroid health. Iodine is essential for the synthesis of thyroid hormones, while selenium is a crucial cofactor for the deiodinase enzymes that regulate the activation and inactivation of these hormones.

Explanation: The Tolerable Upper Intake Level (UL) for selenium for adults is established at 400 micrograms (μg) per day. Consistently exceeding this limit can lead to selenosis, a condition characterized by various toxic effects.

Explanation: Elemental selenium and metallic selenides generally exhibit lower toxicity compared to more bioavailable forms like selenates and selenites, due to their slower absorption rates. High bioavailability correlates with increased toxicity.

Explanation: Biomonitoring of selenium levels in biological samples, such as blood, plasma, serum, or urine, is a standard practice for assessing occupational or environmental exposure and for diagnosing potential selenium toxicity.

Explanation: Selenium's contribution to the immune system is primarily linked to its antioxidant functions via selenoproteins, which help protect immune cells from oxidative stress and modulate immune responses, rather than forming structural components of antibodies.

Explanation: Trace amounts of selenium are biologically essential for humans and other animals, serving as a critical cofactor for various antioxidant enzymes, such as glutathione peroxidase and thioredoxin reductase, which protect cells from oxidative damage.

Explanation: Common symptoms associated with selenium toxicity (selenosis) include a characteristic garlic odor on the breath, hair loss, nail deformities, gastrointestinal disturbances, and neurological effects.

Explanation: Selenium is a vital cofactor for thyroid hormone deiodinase enzymes, which regulate the conversion of the prohormone thyroxine (T4) to the active hormone triiodothyronine (T3) and also facilitate the deactivation of thyroid hormones.

Explanation: Selenium's toxicity is strongly influenced by its bioavailability. Highly bioavailable forms, such as selenates and selenites, are significantly more toxic than elemental selenium or metallic selenides, which are absorbed more slowly.

Explanation: The primary function of selenium in the human body is as a cofactor for antioxidant enzymes, such as glutathione peroxidase and thioredoxin reductase, which play a critical role in protecting cells from oxidative damage.

Explanation: Selenocysteine is unique in that its incorporation into proteins during translation occurs via the UGA codon, which is typically recognized as a stop signal. This process requires specific mRNA structures and translation factors.

Explanation: The Tolerable Upper Intake Level (UL) for selenium for adults is established at 400 micrograms (μg) per day. Exceeding this level increases the risk of adverse health effects.

Explanation: Adequate selenium intake can help mitigate mercury toxicity. Selenium can bind to mercury, forming less toxic complexes, and may support cellular defense mechanisms against mercury's harmful effects.

Explanation: Common dietary sources of selenium include meats, cereals, and nuts. Brazil nuts are particularly notable for their exceptionally high selenium content.

Explanation: Selenium pollution, often originating from agricultural runoff or industrial discharge, can bioaccumulate in aquatic environments, leading to severe health issues and developmental deformities in fish and waterbirds.

Explanation: The Kesterson National Wildlife Refuge in California experienced a significant environmental crisis due to selenium poisoning, primarily caused by agricultural irrigation drainage water concentrating selenium and harming aquatic life and waterbirds.

Explanation: The Globally Harmonized System (GHS) classifies selenium as toxic if swallowed or inhaled, suspected of damaging fertility or the unborn child, and causing organ damage through prolonged exposure. It also carries warnings regarding long-lasting harmful effects on aquatic life.

Explanation: The NFPA 704 rating for selenium indicates a health hazard of 2, flammability of 0 (will not burn), and instability of 0 (normally stable). It does not signify high flammability or significant instability.

Explanation: Certain plant species, termed selenium indicators, possess the ability to thrive in soils with high selenium concentrations and actively accumulate the element, sometimes as a defense mechanism.

Explanation: The Occupational Safety and Health Administration (OSHA) has set the permissible exposure limit (PEL) for selenium in the workplace at 0.2 mg/m³ over an 8-hour time-weighted average.

Explanation: According to the Globally Harmonized System (GHS) classification, selenium is categorized as 'Suspected of damaging fertility or the unborn child' (H361).

Explanation: A significant environmental concern with selenium pollution is its tendency to bioaccumulate in aquatic food webs, leading to elevated concentrations in organisms and causing developmental abnormalities and reproductive issues in fish and waterbirds.

Explanation: The NFPA 704 rating for selenium indicates a flammability hazard of 0, signifying that it will not burn under normal fire conditions.