A chemist is a scientist trained in the study of chemistry. Chemists study the composition of its properties. Chemists describe the properties they study in terms of quantities, with detail on the level of molecules and their component atoms. Chemists measure substance proportions, reaction rates, other chemical properties; the word'chemist' is used to address Pharmacists in Commonwealth English. Chemists use this knowledge to learn the composition and properties of unfamiliar substances, as well as to reproduce and synthesize large quantities of useful occurring substances and create new artificial substances and useful processes. Chemists may specialize in any number of subdisciplines of chemistry. Materials scientists and metallurgists share skills with chemists; the work of chemists is related to the work of chemical engineers, who are concerned with the proper design and evaluation of the most cost-effective large-scale chemical plants and work with industrial chemists on the development of new processes and methods for the commercial-scale manufacture of chemicals and related products.
The roots of chemistry can be traced to the phenomenon of burning. Fire was a mystical force that transformed one substance into another and thus was of primary interest to mankind, it was fire. After gold was discovered and became a precious metal, many people were interested to find a method that could convert other substances into gold; this led to the protoscience called alchemy. The word chemist is derived from an abbreviation of alchimista. Alchemists discovered many chemical processes. Chemistry as we know it today, was invented by Antoine Lavoisier with his law of conservation of mass in 1783; the discoveries of the chemical elements has a long history culminating in the creation of the periodic table by Dmitri Mendeleev. The Nobel Prize in Chemistry created in 1901 gives an excellent overview of chemical discovery since the start of the 20th century. Jobs for chemists require at least a bachelor's degree, but many positions those in research, require a Master of Science or a Doctor of Philosophy.
Most undergraduate programs emphasize mathematics and physics as well as chemistry because chemistry is known as "the central science", thus chemists ought to have a well-rounded knowledge about science. At the Master's level and higher, students tend to specialize in a particular field. Fields of specialization include biochemistry, nuclear chemistry, organic chemistry, inorganic chemistry, polymer chemistry, analytical chemistry, physical chemistry, theoretical chemistry, quantum chemistry, environmental chemistry, thermochemistry. Postdoctoral experience may be required for certain positions. Workers whose work involves chemistry, but not at a complexity requiring an education with a chemistry degree, are referred to as chemical technicians; such technicians do such work as simpler, routine analyses for quality control or in clinical laboratories, having an associate degree. A chemical technologist has more education or experience than a chemical technician but less than a chemist having a bachelor's degree in a different field of science with an associate degree in chemistry or having the same education as a chemical technician but more experience.
There are degrees specific to become a chemical technologist, which are somewhat distinct from those required when a student is interested in becoming a professional chemist. A Chemical technologist is more involved in the management and operation of the equipment and instrumentation necessary to perform chemical analyzes than a chemical technician, they are part of the team of a chemical laboratory in which the quality of the raw material, intermediate products and finished products is analyzed. They perform functions in the areas of environmental quality control and the operational phase of a chemical plant. In addition to all the training given to chemical technologists in their respective degree, a chemist is trained to understand more details related to chemical phenomena so that the chemist can be capable of more planning on the steps to achieve a distinct goal via a chemistry-related endeavor; the higher the competency level achieved in the field of chemistry, the higher the responsibility given to that chemist and the more complicated the task might be.
Chemistry, as a field, have so many applications that different tasks and objectives can be given to workers or scientists with these different levels of education or experience. The specific title of each job varies from position to position, depending on factors such as the kind of industry, the routine level of the task, the current needs of a particular enterprise, the size of the enterprise or hiring firm, the philosophy and management principles of the hiring firm, the visibility of the competency and individual achievements of the one seeking employment, economic factors such as recession or economic depression, among other factors, so this makes it difficult to categorize the exact roles of these chemistry-related workers as standard for that given level of education; because of these factors affecting exact job titles with distinct responsibilities, some chemists might begin doing technician tasks while other chemists might begin doing more complicated tasks than those of a technician, such as tasks th
American Association for the Advancement of Science
The American Association for the Advancement of Science is an American international non-profit organization with the stated goals of promoting cooperation among scientists, defending scientific freedom, encouraging scientific responsibility, supporting scientific education and science outreach for the betterment of all humanity. It is the world's largest general scientific society, with over 120,000 members, is the publisher of the well-known scientific journal Science, which had a weekly circulation of 138,549 in 2008; the American Association for the Advancement of Science was created on September 20, 1848 at the Academy of Natural Sciences in Philadelphia, Pennsylvania. It was a reformation of the Association of American Naturalists; the society chose William Charles Redfield as their first president because he had proposed the most comprehensive plans for the organization. According to the first constitution, agreed to at the September 20 meeting, the goal of the society was to promote scientific dialogue in order to allow for greater scientific collaboration.
By doing so the association aimed to use resources to conduct science with increased efficiency and allow for scientific progress at a greater rate. The association sought to increase the resources available to the scientific community through active advocacy of science. There were only 78 members; as a member of the new scientific body, Matthew Fontaine Maury, USN was one of those who attended the first 1848 meeting. At a meeting held on Friday afternoon, September 22, 1848, Redfield presided, Matthew Fontaine Maury gave a full scientific report on his Wind and Current Charts. Maury stated that hundreds of ship navigators were now sending abstract logs of their voyages to the United States Naval Observatory, he added, "Never before was such a corps of observers known." But, he pointed out to his fellow scientists, his critical need was for more "simultaneous observations." "The work," Maury stated, "is not for the benefit of any nation or age." The minutes of the AAAS meeting reveal that because of the universality of this "view on the subject, it was suggested whether the states of Christendom might not be induced to cooperate with their Navies in the undertaking.
William Barton Rogers, professor at the University of Virginia and founder of the Massachusetts Institute of Technology, offered a resolution: "Resolved that a Committee of five be appointed to address a memorial to the Secretary of the Navy, requesting his further aid in procuring for Matthew Maury the use of the observations of European and other foreign navigators, for the extension and perfecting of his charts of winds and currents." The resolution was adopted and, in addition to Rogers, the following members of the association were appointed to the committee: Professor Joseph Henry of Washington. This was scientific cooperation, Maury went back to Washington with great hopes for the future. By 1860, membership increased to over 2,000; the AAAS became dormant during the American Civil War. The AAAS did not become a permanent casualty of the war. In 1866, Frederick Barnard presided over the first meeting of the resurrected AAAS at a meeting in New York City. Following the revival of the AAAS, the group had considerable growth.
The AAAS permitted all people, regardless of scientific credentials. The AAAS did, institute a policy of granting the title of "Fellow of the AAAS" to well-respected scientists within the organization; the years of peace brought the expansion of other scientific-oriented groups. The AAAS's focus on the unification of many fields of science under a single organization was in contrast to the many new science organizations founded to promote a single discipline. For example, the American Chemical Society, founded in 1876, promotes chemistry. In 1863, the US Congress established the National Academy of Sciences, another multidisciplinary sciences organization, it elects members based on the value of published works. Alan I. Leshner, AAAS CEO from 2001 until 2015, published many op-ed articles discussing how many people integrate science and religion in their lives, he has opposed the insertion of non-scientific content, such as creationism or intelligent design, into the scientific curriculum of schools.
In December 2006, the AAAS adopted an official statement on climate change, in which they stated, "The scientific evidence is clear: global climate change caused by human activities is occurring now, it is a growing threat to society.... The pace of change and the evidence of harm have increased markedly over the last five years; the time to control greenhouse gas emissions is now."In February 2007, the AAAS used satellite images to document human rights abuses in Burma. The next year, AAAS launched the Center for Science Diplomacy to advance both science and the broader relationships among partner countries, by promoting science diplomacy and international scientific cooperation. In 2012, AAAS published op-eds, held events on Capitol Hill and released analyses of the U. S. federal research-and-development budget, to warn that a budget sequestration would have severe consequences for scientific progress. AAAS covers various areas of sciences and engineering, it has twelve sections, each with a committee and its ch
A refrigerant is a substance or mixture a fluid, used in a heat pump and refrigeration cycle. In most cycles it back again. Many working fluids have been used for such purposes. Fluorocarbons chlorofluorocarbons, became commonplace in the 20th century, but they are being phased out because of their ozone depletion effects. Other common refrigerants used in various applications are ammonia, sulfur dioxide, non-halogenated hydrocarbons such as propane; the ideal working fluid or called refrigerant would have favorable thermodynamic properties, be noncorrosive to mechanical components, be safe, including freedom from toxicity and flammability. It would not cause climate change. Since different fluids have the desired traits in different degree, choice is a matter of trade-offs; the desired thermodynamic properties are a boiling point somewhat below the target temperature, a high heat of vaporization, a moderate density in liquid form, a high density in gaseous form, a high critical temperature. Since boiling point and gas density are affected by pressure, refrigerants may be made more suitable for a particular application by appropriate choice of operating pressures.
The inert nature of many haloalkanes, chlorofluorocarbons and hydrochlorofluorocarbons CFC-11 and CFC-12, made them preferred choices among refrigerants for many years because of their non-flammability and non-toxicity. However, their stability in the atmosphere and their corresponding global warming potential and ozone depletion potential raised concerns about their usage; this led to their replacement with HFCs and PFCs HFC-134a, which are not-ozone depleting, have lesser global warming potentials. However, these refrigerants still have global warming potentials thousands of times greater than CO2. Therefore, they are now being replaced in markets where leaks are by using a fourth generation of refrigerants, most prominently HFO-1234yf, which have global warming potentials much closer to that of CO2. In order from the highest to the lowest potential of ozone depletion are: Bromochlorofluorocarbon, CFC HCFC. New refrigerants were developed in the early 21st century that are safer for the environment, but their application has been held up due to concerns over toxicity and flammability.
Compared to halogenated refrigerants, hydrocarbons like isobutane and propane offer several advantages: low cost and available, zero ozone depletion potential and low global warming potential. They have good energy efficiency, but are flammable and can form an explosive mixture with air if a leak occurs. Despite the flammability, they are used in domestic refrigerators. EU and US regulations set maximum charges of 57 or 150 grams of refrigerant, keeping the concentration in a standard kitchen below 20% of the lower explosive limit; the LEL can be exceeded inside the appliance, so no potential ignition sources can be present. Switches must be placed outside the refrigerated compartment or replaced by sealed versions, only spark-free fans can be used. In 2010, about one-third of all household refrigerators and freezers manufactured globally used isobutane or an isobutane/propane blend, this was expected to increase to 75% by 2020. Early mechanical refrigeration systems employed methyl chloride and ammonia.
Being toxic, sulfur dioxide and methyl chloride disappeared from the market with the introduction of CFCs. One may encounter older machines with methyl formate, chloromethane, or dichloromethane. Chlorofluorocarbons were little used for refrigeration until better synthesis methods, developed in the 1950s, reduced their cost, their domination of the market was called into question in the 1980s by concerns about depletion of the ozone layer. Following legislative regulations on ozone depleting chlorofluorocarbons and hydrochlorofluorocarbons, substances used as substitute refrigerants such as perfluorocarbons and hydrofluorocarbons have come under criticism, they are subject to prohibition discussions on account of their harmful effect on the climate. In 1997, FCs and HFCs were included in the Kyoto Protocol to the Framework Convention on Climate Change. In 2006, the EU adopted a Regulation on fluorinated greenhouse gases, which makes stipulations regarding the use of FCs and HFCs with the intention of reducing their emissions.
The provisions do not affect climate-neutral refrigerants. Refrigerants such as ammonia, carbon dioxide and non-halogenated hydrocarbons do not deplete the ozone layer and have no or only a low global warming potential, they are used in air-conditioning systems for buildings, in sport and leisure facilities, in the chemical/pharmaceutical industry, in the automotive industry and above all in the food industry. In these settings their toxicity is less a concern than in home equipment. Emissions from automobile air conditioning are a growing concern because of their impact on climate change. From 2011 on, the European Union will phase out refrigerants with a global warming potential of more than 150 in automotive air conditioning; this will ban potent greenhouse gases such as the refrigerant HFC-134a —which has a GWP of 1410—to promote safe and energy-efficient refrigerants. One of the most promising alternatives is CO2. Carbon dioxide is non-flammable, non-ozone depleting, has a global warming potential of 1.
R-744 can be used as a working flui
Texas is the second largest state in the United States by both area and population. Geographically located in the South Central region of the country, Texas shares borders with the U. S. states of Louisiana to the east, Arkansas to the northeast, Oklahoma to the north, New Mexico to the west, the Mexican states of Chihuahua, Nuevo León, Tamaulipas to the southwest, while the Gulf of Mexico is to the southeast. Houston is the most populous city in Texas and the fourth largest in the U. S. while San Antonio is the second-most populous in the state and seventh largest in the U. S. Dallas–Fort Worth and Greater Houston are the fourth and fifth largest metropolitan statistical areas in the country, respectively. Other major cities include Austin, the second-most populous state capital in the U. S. and El Paso. Texas is nicknamed "The Lone Star State" to signify its former status as an independent republic, as a reminder of the state's struggle for independence from Mexico; the "Lone Star" can be found on the Texan state seal.
The origin of Texas's name is from the word taysha. Due to its size and geologic features such as the Balcones Fault, Texas contains diverse landscapes common to both the U. S. Southern and Southwestern regions. Although Texas is popularly associated with the U. S. southwestern deserts, less than 10% of Texas's land area is desert. Most of the population centers are in areas of former prairies, grasslands and the coastline. Traveling from east to west, one can observe terrain that ranges from coastal swamps and piney woods, to rolling plains and rugged hills, the desert and mountains of the Big Bend; the term "six flags over Texas" refers to several nations. Spain was the first European country to claim the area of Texas. France held a short-lived colony. Mexico controlled the territory until 1836 when Texas won its independence, becoming an independent Republic. In 1845, Texas joined the union as the 28th state; the state's annexation set off a chain of events that led to the Mexican–American War in 1846.
A slave state before the American Civil War, Texas declared its secession from the U. S. in early 1861, joined the Confederate States of America on March 2nd of the same year. After the Civil War and the restoration of its representation in the federal government, Texas entered a long period of economic stagnation. Four major industries shaped the Texas economy prior to World War II: cattle and bison, cotton and oil. Before and after the U. S. Civil War the cattle industry, which Texas came to dominate, was a major economic driver for the state, thus creating the traditional image of the Texas cowboy. In the 19th century cotton and lumber grew to be major industries as the cattle industry became less lucrative, it was though, the discovery of major petroleum deposits that initiated an economic boom which became the driving force behind the economy for much of the 20th century. With strong investments in universities, Texas developed a diversified economy and high tech industry in the mid-20th century.
As of 2015, it is second on the list of the most Fortune 500 companies with 54. With a growing base of industry, the state leads in many industries, including agriculture, energy and electronics, biomedical sciences. Texas has led the U. S. in state export revenue since 2002, has the second-highest gross state product. If Texas were a sovereign state, it would be the 10th largest economy in the world; the name Texas, based on the Caddo word táyshaʼ "friend", was applied, in the spelling Tejas or Texas, by the Spanish to the Caddo themselves the Hasinai Confederacy, the final -s representing the Spanish plural. The Mission San Francisco de los Tejas was completed near the Hasinai village of Nabedaches in May 1690, in what is now Houston County, East Texas. During Spanish colonial rule, in the 18th century, the area was known as Nuevo Reino de Filipinas "New Kingdom of the Philippines", or as provincia de los Tejas "province of the Tejas" also provincia de Texas, "province of Texas", it was incorporated as provincia de Texas into the Mexican Empire in 1821, declared a republic in 1836.
The Royal Spanish Academy recognizes both spellings and Texas, as Spanish-language forms of the name of the U. S. State of Texas; the English pronunciation with /ks/ is unetymological, based in the value of the letter x in historical Spanish orthography. Alternative etymologies of the name advanced in the late 19th century connected the Spanish teja "rooftile", the plural tejas being used to designate indigenous Pueblo settlements. A 1760s map by Jacques-Nicolas Bellin shows a village named Teijas on Trinity River, close to the site of modern Crockett. Texas is the second-largest U. S. state, with an area of 268,820 square miles. Though 10% larger than France and twice as large as Germany or Japan, it ranks only 27th worldwide amongst country subdivisions by size. If it were an independent country, Texas would be the 40th largest behind Zambia. Texas is in the south central part of the United States of America. Three of its borders are defined by rivers; the Rio Grande forms a natural border with the Mexican states of Chihuahua, Nuevo León, Tamaulipas to the south.
The Red River forms a natural border with Arkansas to the north. The Sabine River forms a natural border with Louisiana to the east; the Texas Panhandle has an eastern border with Oklahoma at 100° W, a northern border with Oklahoma at 36°30' N and a western
Carleton College is a private liberal arts college in Northfield, Minnesota. Founded in 1866, the college enrolled 2,105 undergraduate students and employed 269 faculty members in fall 2016; the 200-acre main campus is located between Northfield and the 800-acre Cowling Arboretum, which became part of the campus in the 1920s. In its 2019 edition of national liberal arts college rankings, U. S. News & World Report ranked Carleton fifth-best first for undergraduate teaching. From 2000 through 2016, the institution has produced 122 National Science Graduate Fellows, 112 Fulbright Scholars, 22 Watson Fellows, 20 NCAA Postgraduate Scholars, 13 Goldwater Scholars, 2 Rhodes Scholars. Carleton is one of the largest sources of undergraduate students pursuing doctorates per one hundred students for bachelors institutions; the school was founded in 1866, when the Minnesota Conference of Congregational Churches unanimously accepted a resolution to locate a college in Northfield. Two Northfield businessmen, Charles Augustus Wheaton and Charles Moorehouse Goodsell, each donated 10 acres of land for the first campus.
The first students enrolled at the preparatory unit of Northfield College in the fall of 1867. In 1870, the first college president, James Strong, traveled to the East Coast to raise funds for the college. On his way from visiting a potential donor, William Carleton of Charlestown, Strong was badly injured in a collision between his carriage and a train. Impressed by Strong's survival of the accident, Carleton donated $50,000 to the fledgling institution in 1871; as a result, the Board of Trustees renamed the school in his honor. The college graduated its first college class in 1874, James J. Dow and Myra A. Brown, who married each other that year. On September 7, 1876, the James-Younger Gang, led by outlaw Jesse James, tried to rob the First National Bank of Northfield. Joseph Lee Heywood, Carleton's Treasurer, was acting cashier at the bank that day, he was killed for refusing to open the safe. Carleton named a library fund after Heywood; the Heywood Society is the name for a group of donors. In its early years under the presidency of James Strong, Carleton reflected the theological conservatism of its Minnesota Congregational founders.
In 1903, modern religious influences were introduced by William Sallmon, a Yale Divinity School graduate, hired as college president. Sallmon was opposed by conservative faculty members and alumni, left the presidency by 1908. After Sallmon left, the trustees hired Donald J. Cowling, another theologically liberal Yale Divinity School graduate, as his successor. In 1916, under Cowling's leadership, Carleton began an official affiliation with the Minnesota Baptist Convention, it lasted until 1928, when the Baptists severed the relationship as a result of fundamentalist opposition to Carleton's liberalism, including the college's support for teaching evolution. Non-denominational for a number of years, in 1964 Carleton abolished its requirement for weekly attendance at some religious or spiritual meeting. In 1927, students founded the first student-run pub in The Cave. Located in the basement of Evans Hall, it continues to host live music shows and other events several times each week. In 1942, Carleton purchased land in Stanton, about 10 miles east of campus, to use for flight training.
During World War II, several classes of male students went through air basic training at the college. Since being sold by the college in 1944, the Stanton Airfield has been operated for commercial use; the world premiere production of the English translation of Bertolt Brecht's play, The Caucasian Chalk Circle, was performed in 1948 at Carleton's Nourse Little Theater. In 1963 the Reformed Druids of North America was founded by students at Carleton as a means to be excused from attendance of then-mandatory weekly chapel service. Within a few years, the group evolved to engage in legitimate spiritual exploration. Meetings continue to be held in the Carleton College Cowling Arboretum. President Bill Clinton gave the last commencement address of his administration at Carleton, on June 10, 2000, marking the first presidential visit to the college. Carleton is a small, liberal arts college offering 33 different majors and 31 minors, is accredited by the Higher Learning Commission. Students have the option to design their own major.
There are ten languages offered: Spanish, German, Japanese, Arabic, Latin and Hebrew. The academic calendar follows a trimester system where students take three classes per 10-week term. In order to graduate with a degree from Carleton, students must take an Argument & Inquiry Seminar in their first year, a writing course, three quantitative reasoning encounters, international studies, intercultural domestic studies, humanistic inquiry, literary/artistic analysis, arts practice, formal or statistical reasoning, social inquiry, physical education; the average class size at Carleton is 16. 48% have 10–19 students, 24% of all classes have 2–9 students, 21% have 20–29 students, 5% have 30 or more students. The most popular areas of study are biology, political science and international relations, chemistry, psychology and computer science. Carleton is one of the few liberal arts colleges. Studying abroad is common at Carleton: 76% of the senior class of 2018 studied abroad at least once over their four years.
Carleton offers a number of its own programs each year, which are led by Carleton faculty and available only to Carleton students. In 2017-2018 there were 17 of such programs offered. Although m
Cornell University is a private and statutory Ivy League research university in Ithaca, New York. Founded in 1865 by Ezra Cornell and Andrew Dickson White, the university was intended to teach and make contributions in all fields of knowledge—from the classics to the sciences, from the theoretical to the applied; these ideals, unconventional for the time, are captured in Cornell's founding principle, a popular 1868 Ezra Cornell quotation: "I would found an institution where any person can find instruction in any study."The university is broadly organized into seven undergraduate colleges and seven graduate divisions at its main Ithaca campus, with each college and division defining its own admission standards and academic programs in near autonomy. The university administers two satellite medical campuses, one in New York City and one in Education City and Cornell Tech, a graduate program that incorporates technology and creative thinking; the program moved from Google's Chelsea Building in New York City to its permanent campus on Roosevelt Island in September 2017.
Cornell is one of ten private land grant universities in the United States and the only one in New York. Of its seven undergraduate colleges, three are state-supported statutory or contract colleges through the State University of New York system, including its agricultural and human ecology colleges as well as its industrial labor relations school. Of Cornell's graduate schools, only the veterinary college is state-supported; as a land grant college, Cornell operates a cooperative extension outreach program in every county of New York and receives annual funding from the State of New York for certain educational missions. The Cornell University Ithaca Campus comprises 745 acres, but is much larger when the Cornell Botanic Gardens and the numerous university-owned lands in New York City are considered; as of October 2018, 58 Nobel laureates, four Turing Award winners and one Fields Medalist have been affiliated with Cornell University. Since its founding, Cornell has been a co-educational, non-sectarian institution where admission has not been restricted by religion or race.
Cornell counts more than 245,000 living alumni, its former and present faculty and alumni include 34 Marshall Scholars, 30 Rhodes Scholars, 29 Truman Scholars, 7 Gates Scholars, 55 Olympic Medalists, 14 living billionaires. The student body consists of more than 14,000 undergraduate and 8,000 graduate students from all 50 American states and 116 countries. Cornell University was founded on April 27, 1865. Senator Ezra Cornell offered his farm in Ithaca, New York, as a site and $500,000 of his personal fortune as an initial endowment. Fellow senator and educator Andrew Dickson White agreed to be the first president. During the next three years, White oversaw the construction of the first two buildings and traveled to attract students and faculty; the university was inaugurated on October 7, 1868, 412 men were enrolled the next day. Cornell developed as a technologically innovative institution, applying its research to its own campus and to outreach efforts. For example, in 1883 it was one of the first university campuses to use electricity from a water-powered dynamo to light the grounds.
Since 1894, Cornell fulfill statutory requirements. Cornell has had active alumni since its earliest classes, it was one of the first universities to include alumni-elected representatives on its Board of Trustees. Cornell was among the Ivies that had heightened student activism during the 1960s related to cultural issues, civil rights, opposition to the Vietnam War. Today the university has more than 4,000 courses. Cornell is known for the Residential Club Fire of 1967, a fire in the Residential Club building that killed eight students and one professor. Since 2000, Cornell has been expanding its international programs. In 2004, the university opened the Weill Cornell Medical College in Qatar, it has partnerships with institutions in India and the People's Republic of China. Former president Jeffrey S. Lehman described the university, with its high international profile, a "transnational university". On March 9, 2004, Cornell and Stanford University laid the cornerstone for a new'Bridging the Rift Center' to be built and jointly operated for education on the Israel–Jordan border.
Cornell's main campus is on East Hill in Ithaca, New York, overlooking Cayuga Lake. Since the university was founded, it has expanded to about 2,300 acres, encompassing both the hill and much of the surrounding areas. Central Campus has laboratories, administrative buildings, all of the campus' academic buildings, athletic facilities and museums. North Campus is composed of ten residence halls that house first-year students, although the Townhouse Community houses transfer students; the five main residence halls on West Campus make up the West Campus House System, along with several Gothic-style buildings, referred to as "the Gothics". Collegetown contains two upper-level residence halls and the Schwartz Performing Arts Center amid a mixed-use neighborhood of apartments and businesses; the main campus is marked by an irregular layout and eclectic architectural styles, including ornate Collegiate Gothic and Neoclassical buildings, the more spare international and modernist structures. The more ornat
Hydrogen is a chemical element with symbol H and atomic number 1. With a standard atomic weight of 1.008, hydrogen is the lightest element in the periodic table. Hydrogen is the most abundant chemical substance in the Universe, constituting 75% of all baryonic mass. Non-remnant stars are composed of hydrogen in the plasma state; the most common isotope of hydrogen, termed protium, has no neutrons. The universal emergence of atomic hydrogen first occurred during the recombination epoch. At standard temperature and pressure, hydrogen is a colorless, tasteless, non-toxic, nonmetallic combustible diatomic gas with the molecular formula H2. Since hydrogen forms covalent compounds with most nonmetallic elements, most of the hydrogen on Earth exists in molecular forms such as water or organic compounds. Hydrogen plays a important role in acid–base reactions because most acid-base reactions involve the exchange of protons between soluble molecules. In ionic compounds, hydrogen can take the form of a negative charge when it is known as a hydride, or as a positively charged species denoted by the symbol H+.
The hydrogen cation is written as though composed of a bare proton, but in reality, hydrogen cations in ionic compounds are always more complex. As the only neutral atom for which the Schrödinger equation can be solved analytically, study of the energetics and bonding of the hydrogen atom has played a key role in the development of quantum mechanics. Hydrogen gas was first artificially produced in the early 16th century by the reaction of acids on metals. In 1766–81, Henry Cavendish was the first to recognize that hydrogen gas was a discrete substance, that it produces water when burned, the property for which it was named: in Greek, hydrogen means "water-former". Industrial production is from steam reforming natural gas, less from more energy-intensive methods such as the electrolysis of water. Most hydrogen is used near the site of its production, the two largest uses being fossil fuel processing and ammonia production for the fertilizer market. Hydrogen is a concern in metallurgy as it can embrittle many metals, complicating the design of pipelines and storage tanks.
Hydrogen gas is flammable and will burn in air at a wide range of concentrations between 4% and 75% by volume. The enthalpy of combustion is −286 kJ/mol: 2 H2 + O2 → 2 H2O + 572 kJ Hydrogen gas forms explosive mixtures with air in concentrations from 4–74% and with chlorine at 5–95%; the explosive reactions may be triggered by heat, or sunlight. The hydrogen autoignition temperature, the temperature of spontaneous ignition in air, is 500 °C. Pure hydrogen-oxygen flames emit ultraviolet light and with high oxygen mix are nearly invisible to the naked eye, as illustrated by the faint plume of the Space Shuttle Main Engine, compared to the visible plume of a Space Shuttle Solid Rocket Booster, which uses an ammonium perchlorate composite; the detection of a burning hydrogen leak may require a flame detector. Hydrogen flames in other conditions are blue; the destruction of the Hindenburg airship was a notorious example of hydrogen combustion and the cause is still debated. The visible orange flames in that incident were the result of a rich mixture of hydrogen to oxygen combined with carbon compounds from the airship skin.
H2 reacts with every oxidizing element. Hydrogen can react spontaneously and violently at room temperature with chlorine and fluorine to form the corresponding hydrogen halides, hydrogen chloride and hydrogen fluoride, which are potentially dangerous acids; the ground state energy level of the electron in a hydrogen atom is −13.6 eV, equivalent to an ultraviolet photon of 91 nm wavelength. The energy levels of hydrogen can be calculated accurately using the Bohr model of the atom, which conceptualizes the electron as "orbiting" the proton in analogy to the Earth's orbit of the Sun. However, the atomic electron and proton are held together by electromagnetic force, while planets and celestial objects are held by gravity; because of the discretization of angular momentum postulated in early quantum mechanics by Bohr, the electron in the Bohr model can only occupy certain allowed distances from the proton, therefore only certain allowed energies. A more accurate description of the hydrogen atom comes from a purely quantum mechanical treatment that uses the Schrödinger equation, Dirac equation or the Feynman path integral formulation to calculate the probability density of the electron around the proton.
The most complicated treatments allow for the small effects of special relativity and vacuum polarization. In the quantum mechanical treatment, the electron in a ground state hydrogen atom has no angular momentum at all—illustrating how the "planetary orbit" differs from electron motion. There exist two different spin isomers of hydrogen diatomic molecules that differ by the relative spin of their nuclei. In the orthohydrogen form, the spins of the two protons are parallel and form a triplet state with a molecular spin quantum number of 1. At standard temperature and pressure, hydrogen gas contains about 25% of the para form and 75% of the ortho form known as the "normal form"; the equilibrium ratio of orthohydrogen to parahydrogen depends on temperature, but because the ortho form is an excited state and has a higher energy