A semiconductor material has an electrical conductivity value falling between that of a metal, like copper, etc. and an insulator, such as glass. Their resistance decreases as their temperature increases, behaviour opposite to that of a metal, their conducting properties may be altered in useful ways by the deliberate, controlled introduction of impurities into the crystal structure. Where two differently-doped regions exist in the same crystal, a semiconductor junction is created; the behavior of charge carriers which include electrons and electron holes at these junctions is the basis of diodes and all modern electronics. Some examples of semiconductors are silicon and gallium arsenide. After silicon, gallium arsenide is the second most common semiconductor used in laser diodes, solar cells, microwave frequency integrated circuits, others. Silicon is a critical element for fabricating most electronic circuits. Semiconductor devices can display a range of useful properties such as passing current more in one direction than the other, showing variable resistance, sensitivity to light or heat.
Because the electrical properties of a semiconductor material can be modified by doping, or by the application of electrical fields or light, devices made from semiconductors can be used for amplification and energy conversion. The conductivity of silicon is increased by adding a small amount of trivalent atoms; this process is known as doping and resulting semiconductors are known as doped or extrinsic semiconductors. Apart from doping, the conductivity of a semiconductor can be improved by increasing its temperature; this is contrary to the behaviour of a metal in which conductivity decreases with increase in temperature. The modern understanding of the properties of a semiconductor relies on quantum physics to explain the movement of charge carriers in a crystal lattice. Doping increases the number of charge carriers within the crystal; when a doped semiconductor contains free holes it is called "p-type", when it contains free electrons it is known as "n-type". The semiconductor materials used in electronic devices are doped under precise conditions to control the concentration and regions of p- and n-type dopants.
A single semiconductor crystal can have many p- and n-type regions. Although some pure elements and many compounds display semiconductor properties, silicon and compounds of gallium are the most used in electronic devices. Elements near the so-called "metalloid staircase", where the metalloids are located on the periodic table, are used as semiconductors; some of the properties of semiconductor materials were observed throughout the mid 19th and first decades of the 20th century. The first practical application of semiconductors in electronics was the 1904 development of the cat's-whisker detector, a primitive semiconductor diode used in early radio receivers. Developments in quantum physics in turn allowed the development of the transistor in 1947 and the integrated circuit in 1958. Variable electrical conductivity Semiconductors in their natural state are poor conductors because a current requires the flow of electrons, semiconductors have their valence bands filled, preventing the entry flow of new electrons.
There are several developed techniques that allow semiconducting materials to behave like conducting materials, such as doping or gating. These modifications have two outcomes: p-type; these refer to the shortage of electrons, respectively. An unbalanced number of electrons would cause a current to flow through the material. Heterojunctions Heterojunctions occur when two differently doped semiconducting materials are joined together. For example, a configuration could consist of n-doped germanium; this results in an exchange of electrons and holes between the differently doped semiconducting materials. The n-doped germanium would have an excess of electrons, the p-doped germanium would have an excess of holes; the transfer occurs until equilibrium is reached by a process called recombination, which causes the migrating electrons from the n-type to come in contact with the migrating holes from the p-type. A product of this process is charged ions. Excited electrons A difference in electric potential on a semiconducting material would cause it to leave thermal equilibrium and create a non-equilibrium situation.
This introduces electrons and holes to the system, which interact via a process called ambipolar diffusion. Whenever thermal equilibrium is disturbed in a semiconducting material, the number of holes and electrons changes; such disruptions can occur as a result of a temperature difference or photons, which can enter the system and create electrons and holes. The process that creates and annihilates electrons and holes are called generation and recombination. Light emission In certain semiconductors, excited electrons can relax by emitting light instead of producing heat; these semiconductors are used in the construction of light-emitting diodes and fluorescent quantum dots. High thermal conductivitySemiconductors with high thermal conductivity can be used for heat dissipation and improving thermal management of electronics. Thermal energy conversion Semiconductors have large thermoelectric power factors making them useful in thermoelectric generators, as well as high thermoelectric figures of merit making them useful in thermoelectric coolers.
A large number of elements and compounds have semiconducting properties, including: Certain pure elements are found in Group 14 of the p
George Porter, Baron Porter of Luddenham, was a British chemist. He was awarded the Nobel Prize in Chemistry in 1967. Porter was born near Thorne, South Yorkshire, he was educated at Thorne Grammar School won a scholarship to the University of Leeds and gained his first degree in chemistry. He was awarded a PhD from the University of Cambridge in 1949 for research investigating free radicals produced by photochemical means. Porter served in the Royal Naval Volunteer Reserve during the Second World War. Porter went on to do research at the University of Cambridge supervised by Ronald George Wreyford Norrish where he began the work that led to them becoming Nobel Laureates, his original research in developing the technique of Flash photolysis to obtain information on short-lived molecular species provided the first evidence of free Radicals. His research utilised the technique to study the minutiae of the Light-dependent reactions of Photosynthesis, with particular regard to possible applications to a Hydrogen economy, of which he was a strong advocate.
He was Assistant Director of the British Rayon Research Association from 1953–4, where he studied the phototendering of dyed cellulose fabrics in sunlight. Porter became a professor in the Chemistry department at the University of Sheffield in 1954-55, it was here he started his work on Flash Photolysis with equipment designed and made in the departmental workshop. During this tenure he took part in a television programme describing his work; this was in the "Eye on Research" series. Porter became Fullerian Professor of Chemistry and Director of the Royal Institution in 1966. During his directorship of the Royal Institution, Porter was instrumental in the setting up of Applied Photophysics, a company created to supply instrumentation based on his group's work, he was awarded the Nobel Prize in Chemistry in 1967 along with Manfred Eigen and Ronald George Wreyford Norrish. In the same year he became a Visiting Professor at University College London. Porter was a major contributor to the Public Understanding of science.
He became president of the British Association in 1985 and was the founding Chair of the Committee on the Public Understanding of Science. He gave the Romanes Lecture, entitled "Science and the human purpose", at the University of Oxford in 1978. From 1990 to 1993 he gave. Porter was elected a Fellow of the Royal Society in 1960 and served as President of the Royal Society from 1985–1990, he was awarded the Davy Medal in 1971, the Rumford Medal in 1978, the Ellison-Cliffe Medal in 1991 and the Copley Medal in 1992. Porter received an Honorary Doctorate from Heriot-Watt University in 1971, he was knighted in 1972, appointed to the Order of Merit in 1989, was made a life peer as Baron Porter of Luddenham, of Luddenham in the County of Kent, in 1990. In 1995, he was awarded an Honorary Degree from the University of Bath. In 1976 he gave the Royal Institution Christmas Lecture on The Natural History of a Sunbeam. Porter served as Chancellor of the University of Leicester between 1984 and 1995. In 2001, the university's chemistry building was named the George Porter Building in his honour.
In 1949 he married Stella Jean Brooke. Chemistry for the Modern World Chemistry in Microtime Profile – Royal Institution of Great Britain The Life and Scientific Legacy of George Porter, World Scientific Publishing, 2006 Obituary in The Guardian, 3 September 2002 Biographical Database of the British Chemical Community, 1880–1970 "The Relevance of Science". George Porter. JASA Vol. 28. March 1976. Pp. 2–3
Nanoparticles are particles between 1 and 100 nanometres in size with a surrounding interfacial layer. The interfacial layer is an integral part of nanoscale matter, fundamentally affecting all of its properties; the interfacial layer consists of ions and organic molecules. Organic molecules coating inorganic nanoparticles are known as stabilizers and surface ligands, or passivating agents. In nanotechnology, a particle is defined as a small object that behaves as a whole unit with respect to its transport and properties. Particles are further classified according to diameter; the term "nanoparticle" is not applied to individual molecules. Ultrafine particles are the same as nanoparticles and between 1 and 100 nm in size, as opposed to fine particles are sized between 100 and 2,500 nm, coarse particles cover a range between 2,500 and 10,000 nm; the reason for the synonymous definition of nanoparticles and ultrafine particles is that, during the 1970s and 80s, when the first thorough fundamental studies with "nanoparticles" were underway in the USA and Japan, they were called "ultrafine particles".
However, during the 1990s before the National Nanotechnology Initiative was launched in the USA, the new name, "nanoparticle," had become more common. Nanoparticles can exhibit size-related properties different from those of either fine particles or bulk materials. Nanoclusters have at least one dimension a narrow size distribution. Nanopowders nanoparticles, or nanoclusters. Nanometer-sized single crystals, or single-domain ultrafine particles, are referred to as nanocrystals. According to ISO Technical Specification 80004, a nanoparticle is defined as a nano-object with all three external dimensions in the nanoscale, whose longest and shortest axes do not differ with a significant difference being a factor of at least 3; the terms colloid and nanoparticle are not interchangeable. A colloid is a mixture; the term applies only if the particles are larger than atomic dimensions but small enough to exhibit Brownian motion, with the critical size range ranging from nanometers to micrometers. Colloids can contain particles too large to be nanoparticles, nanoparticles can exist in non-colloidal form, for examples as a powder or in a solid matrix.
Although nanoparticles are associated with modern science, they have a long history. Nanoparticles were used by artisans as far back as Rome in the fourth century in the famous Lycurgus cup made of dichroic glass as well as the ninth century in Mesopotamia for creating a glittering effect on the surface of pots. In modern times, pottery from the Middle Ages and Renaissance retains a distinct gold- or copper-colored metallic glitter; this luster is caused by a metallic film, applied to the transparent surface of a glazing. The luster can still be visible if the film has resisted other weathering; the luster originates within the film itself, which contains silver and copper nanoparticles dispersed homogeneously in the glassy matrix of the ceramic glaze. These nanoparticles are created by the artisans by adding copper and silver salts and oxides together with vinegar and clay on the surface of previously-glazed pottery; the object is placed into a kiln and heated to about 600 °C in a reducing atmosphere.
In heat the glaze softens, causing the copper and silver ions to migrate into the outer layers of the glaze. There the reducing atmosphere reduced the ions back to metals, which came together forming the nanoparticles that give the color and optical effects. Luster technique showed; the technique originated in the Muslim world. As Muslims were not allowed to use gold in artistic representations, they sought a way to create a similar effect without using real gold; the solution they found was using luster. Michael Faraday provided the first description, in scientific terms, of the optical properties of nanometer-scale metals in his classic 1857 paper. In a subsequent paper, the author points out that: "It is well known that when thin leaves of gold or silver are mounted upon glass and heated to a temperature, well below a red heat, a remarkable change of properties takes place, whereby the continuity of the metallic film is destroyed; the result is that white light is now transmitted, reflection is correspondingly diminished, while the electrical resistivity is enormously increased."
Nanoparticles are of great scientific interest as they are, in effect, a bridge between bulk materials and atomic or molecular structures. A bulk material should have constant physical properties regardless of its size, but at the nano-scale size-dependent properties are observed. Thus, the properties of materials change as their size approaches the nanoscale and as the percentage of the surface in relation to the percentage of the volume of a material becomes significant. For bulk materials larger than one micrometer, the percentage of the surface is insignificant in relation to the volume in the bulk of the material; the interesting and sometimes unexpected properties of nanoparticles are therefore due to the large surface area of the material, which dominates the contributions made by the small bulk of the material. Nanoparticles possess unexpected optical properties as they are small enough
Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that can be released to fuel the organisms' activities. This chemical energy is stored in carbohydrate molecules, such as sugars, which are synthesized from carbon dioxide and water – hence the name photosynthesis, from the Greek φῶς, phōs, "light", σύνθεσις, synthesis, "putting together". In most cases, oxygen is released as a waste product. Most plants, most algae, cyanobacteria perform photosynthesis. Photosynthesis is responsible for producing and maintaining the oxygen content of the Earth's atmosphere, supplies all of the organic compounds and most of the energy necessary for life on Earth. Although photosynthesis is performed differently by different species, the process always begins when energy from light is absorbed by proteins called reaction centres that contain green chlorophyll pigments. In plants, these proteins are held inside organelles called chloroplasts, which are most abundant in leaf cells, while in bacteria they are embedded in the plasma membrane.
In these light-dependent reactions, some energy is used to strip electrons from suitable substances, such as water, producing oxygen gas. The hydrogen freed by the splitting of water is used in the creation of two further compounds that serve as short-term stores of energy, enabling its transfer to drive other reactions: these compounds are reduced nicotinamide adenine dinucleotide phosphate and adenosine triphosphate, the "energy currency" of cells. In plants and cyanobacteria, long-term energy storage in the form of sugars is produced by a subsequent sequence of light-independent reactions called the Calvin cycle. In the Calvin cycle, atmospheric carbon dioxide is incorporated into existing organic carbon compounds, such as ribulose bisphosphate. Using the ATP and NADPH produced by the light-dependent reactions, the resulting compounds are reduced and removed to form further carbohydrates, such as glucose; the first photosynthetic organisms evolved early in the evolutionary history of life and most used reducing agents such as hydrogen or hydrogen sulfide, rather than water, as sources of electrons.
Cyanobacteria appeared later. Today, the average rate of energy capture by photosynthesis globally is 130 terawatts, about eight times the current power consumption of human civilization. Photosynthetic organisms convert around 100–115 billion tonnes of carbon into biomass per year. Photosynthetic organisms are photoautotrophs, which means that they are able to synthesize food directly from carbon dioxide and water using energy from light. However, not all organisms use carbon dioxide as a source of carbon atoms to carry out photosynthesis. In plants and cyanobacteria, photosynthesis releases oxygen; this is called oxygenic photosynthesis and is by far the most common type of photosynthesis used by living organisms. Although there are some differences between oxygenic photosynthesis in plants and cyanobacteria, the overall process is quite similar in these organisms. There are many varieties of anoxygenic photosynthesis, used by certain types of bacteria, which consume carbon dioxide but do not release oxygen.
Carbon dioxide is converted into sugars in a process called carbon fixation. Carbon fixation is an endothermic redox reaction. In general outline, photosynthesis is the opposite of cellular respiration: while photosynthesis is a process of reduction of carbon dioxide to carbohydrate, cellular respiration is the oxidation of carbohydrate or other nutrients to carbon dioxide. Nutrients used in cellular respiration include amino acids and fatty acids; these nutrients are oxidized to produce carbon dioxide and water, to release chemical energy to drive the organism's metabolism. Photosynthesis and cellular respiration are distinct processes, as they take place through different sequences of chemical reactions and in different cellular compartments; the general equation for photosynthesis as first proposed by Cornelis van Niel is therefore: CO2carbondioxide + 2H2Aelectron donor + photonslight energy → carbohydrate + 2Aoxidizedelectrondonor + H2OwaterSince water is used as the electron donor in oxygenic photosynthesis, the equation for this process is: CO2carbondioxide + 2H2Owater + photonslight energy → carbohydrate + O2oxygen + H2OwaterThis equation emphasizes that water is both a reactant in the light-dependent reaction and a product of the light-independent reaction, but canceling n water molecules from each side gives the net equation: CO2carbondioxide + H2O water + photonslight energy → carbohydrate + O2 oxygen Other processes substitute other compounds for water in the electron-supply role.
In the first stage, light-dependent reactions or light reactions capture the energy of light and use it to make the energy-storage molecules ATP and NADPH. During the second stage, the light-independent reactions use these products to capture and reduce carbon dioxid
World War II
World War II known as the Second World War, was a global war that lasted from 1939 to 1945. The vast majority of the world's countries—including all the great powers—eventually formed two opposing military alliances: the Allies and the Axis. A state of total war emerged, directly involving more than 100 million people from over 30 countries; the major participants threw their entire economic and scientific capabilities behind the war effort, blurring the distinction between civilian and military resources. World War II was the deadliest conflict in human history, marked by 50 to 85 million fatalities, most of whom were civilians in the Soviet Union and China, it included massacres, the genocide of the Holocaust, strategic bombing, premeditated death from starvation and disease, the only use of nuclear weapons in war. Japan, which aimed to dominate Asia and the Pacific, was at war with China by 1937, though neither side had declared war on the other. World War II is said to have begun on 1 September 1939, with the invasion of Poland by Germany and subsequent declarations of war on Germany by France and the United Kingdom.
From late 1939 to early 1941, in a series of campaigns and treaties, Germany conquered or controlled much of continental Europe, formed the Axis alliance with Italy and Japan. Under the Molotov–Ribbentrop Pact of August 1939, Germany and the Soviet Union partitioned and annexed territories of their European neighbours, Finland and the Baltic states. Following the onset of campaigns in North Africa and East Africa, the fall of France in mid 1940, the war continued between the European Axis powers and the British Empire. War in the Balkans, the aerial Battle of Britain, the Blitz, the long Battle of the Atlantic followed. On 22 June 1941, the European Axis powers launched an invasion of the Soviet Union, opening the largest land theatre of war in history; this Eastern Front trapped most crucially the German Wehrmacht, into a war of attrition. In December 1941, Japan launched a surprise attack on the United States as well as European colonies in the Pacific. Following an immediate U. S. declaration of war against Japan, supported by one from Great Britain, the European Axis powers declared war on the U.
S. in solidarity with their Japanese ally. Rapid Japanese conquests over much of the Western Pacific ensued, perceived by many in Asia as liberation from Western dominance and resulting in the support of several armies from defeated territories; the Axis advance in the Pacific halted in 1942. Key setbacks in 1943, which included a series of German defeats on the Eastern Front, the Allied invasions of Sicily and Italy, Allied victories in the Pacific, cost the Axis its initiative and forced it into strategic retreat on all fronts. In 1944, the Western Allies invaded German-occupied France, while the Soviet Union regained its territorial losses and turned toward Germany and its allies. During 1944 and 1945 the Japanese suffered major reverses in mainland Asia in Central China, South China and Burma, while the Allies crippled the Japanese Navy and captured key Western Pacific islands; the war in Europe concluded with an invasion of Germany by the Western Allies and the Soviet Union, culminating in the capture of Berlin by Soviet troops, the suicide of Adolf Hitler and the German unconditional surrender on 8 May 1945.
Following the Potsdam Declaration by the Allies on 26 July 1945 and the refusal of Japan to surrender under its terms, the United States dropped atomic bombs on the Japanese cities of Hiroshima and Nagasaki on 6 and 9 August respectively. With an invasion of the Japanese archipelago imminent, the possibility of additional atomic bombings, the Soviet entry into the war against Japan and its invasion of Manchuria, Japan announced its intention to surrender on 15 August 1945, cementing total victory in Asia for the Allies. Tribunals were set up by fiat by the Allies and war crimes trials were conducted in the wake of the war both against the Germans and the Japanese. World War II changed the political social structure of the globe; the United Nations was established to foster international co-operation and prevent future conflicts. The Soviet Union and United States emerged as rival superpowers, setting the stage for the nearly half-century long Cold War. In the wake of European devastation, the influence of its great powers waned, triggering the decolonisation of Africa and Asia.
Most countries whose industries had been damaged moved towards economic expansion. Political integration in Europe, emerged as an effort to end pre-war enmities and create a common identity; the start of the war in Europe is held to be 1 September 1939, beginning with the German invasion of Poland. The dates for the beginning of war in the Pacific include the start of the Second Sino-Japanese War on 7 July 1937, or the Japanese invasion of Manchuria on 19 September 1931. Others follow the British historian A. J. P. Taylor, who held that the Sino-Japanese War and war in Europe and its colonies occurred and the two wars merged in 1941; this article uses the conventional dating. Other starting dates sometimes used for World War II include the Italian invasion of Abyssinia on 3 October 1935; the British historian Antony Beevor views the beginning of World War II as the Battles of Khalkhin Gol fought between Japan and the fo
The electric flash-lamp uses electric current to start flash powder burning, to provide a brief sudden burst of bright light. It had other uses as well. Photographers' flash powder, introduced in 1887 by Adolf Miethe and Johannes Gaedicke, had to be ignited manually, exposing the user to greater risk; the electric flash-lamp was invented by Joshua Cohen in 1899, by Paul Boyer in France. It was granted U. S. patent number 636,492. This flash of bright light from the flash-lamp was used for indoor photography in the late nineteenth century and the early part of the twentieth century. Joshua Lionel Cohen's flash-lamp patent 636,492 reads in part, The principle of operation of the electrical flash-lamp is linked to the shutter of an early box camera: tripping the shutter ignites the flash powder and releases the potential energy of the exploding powder causing a bright flash for indoor photography; the main purpose of Cohen's invention was as a fuse to ignite explosive powder to get a photographer's flash.
One of the first practical applications, for Cohen's flash-lamp was as underwater mine detonator fuses for the U. S. Navy. In 1899, the year the invention was patented, the government awarded Cohen a $12,000 contract for 24,000 naval mine detonator fuses; the use of the flash for photography was dangerous, photographers could get burned hands from the flash. A 1910 brochure for the Nesbit High Speed Flashlight Apparatus says, Valentin Wolfenstein Flash Flashlight Beyer, The Greatest Stories Never Told - 100 Tales from History to Astonish, Bewilder & Stupefy, The History Channel, 2000, ISBN 0-06-001401-6
A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking of chemical bonds between atoms, with no change to the nuclei, can be described by a chemical equation. Nuclear chemistry is a sub-discipline of chemistry that involves the chemical reactions of unstable and radioactive elements where both electronic and nuclear changes can occur; the substance involved in a chemical reaction are called reactants or reagents. Chemical reactions are characterized by a chemical change, they yield one or more products, which have properties different from the reactants. Reactions consist of a sequence of individual sub-steps, the so-called elementary reactions, the information on the precise course of action is part of the reaction mechanism. Chemical reactions are described with chemical equations, which symbolically present the starting materials, end products, sometimes intermediate products and reaction conditions.
Chemical reactions happen at a characteristic reaction rate at a given temperature and chemical concentration. Reaction rates increase with increasing temperature because there is more thermal energy available to reach the activation energy necessary for breaking bonds between atoms. Reactions may proceed in the forward or reverse direction until they go to completion or reach equilibrium. Reactions that proceed in the forward direction to approach equilibrium are described as spontaneous, requiring no input of free energy to go forward. Non-spontaneous reactions require input of free energy to go forward. Different chemical reactions are used in combinations during chemical synthesis in order to obtain a desired product. In biochemistry, a consecutive series of chemical reactions form metabolic pathways; these reactions are catalyzed by protein enzymes. Enzymes increase the rates of biochemical reactions, so that metabolic syntheses and decompositions impossible under ordinary conditions can occur at the temperatures and concentrations present within a cell.
The general concept of a chemical reaction has been extended to reactions between entities smaller than atoms, including nuclear reactions, radioactive decays, reactions between elementary particles, as described by quantum field theory. Chemical reactions such as combustion in fire and the reduction of ores to metals were known since antiquity. Initial theories of transformation of materials were developed by Greek philosophers, such as the Four-Element Theory of Empedocles stating that any substance is composed of the four basic elements – fire, water and earth. In the Middle Ages, chemical transformations were studied by Alchemists, they attempted, in particular, to convert lead into gold, for which purpose they used reactions of lead and lead-copper alloys with sulfur. The production of chemical substances that do not occur in nature has long been tried, such as the synthesis of sulfuric and nitric acids attributed to the controversial alchemist Jābir ibn Hayyān; the process involved heating of sulfate and nitrate minerals such as copper sulfate and saltpeter.
In the 17th century, Johann Rudolph Glauber produced hydrochloric acid and sodium sulfate by reacting sulfuric acid and sodium chloride. With the development of the lead chamber process in 1746 and the Leblanc process, allowing large-scale production of sulfuric acid and sodium carbonate chemical reactions became implemented into the industry. Further optimization of sulfuric acid technology resulted in the contact process in the 1880s, the Haber process was developed in 1909–1910 for ammonia synthesis. From the 16th century, researchers including Jan Baptist van Helmont, Robert Boyle, Isaac Newton tried to establish theories of the experimentally observed chemical transformations; the phlogiston theory was proposed in 1667 by Johann Joachim Becher. It postulated the existence of a fire-like element called "phlogiston", contained within combustible bodies and released during combustion; this proved to be false in 1785 by Antoine Lavoisier who found the correct explanation of the combustion as reaction with oxygen from the air.
Joseph Louis Gay-Lussac recognized in 1808 that gases always react in a certain relationship with each other. Based on this idea and the atomic theory of John Dalton, Joseph Proust had developed the law of definite proportions, which resulted in the concepts of stoichiometry and chemical equations. Regarding the organic chemistry, it was long believed that compounds obtained from living organisms were too complex to be obtained synthetically. According to the concept of vitalism, organic matter was endowed with a "vital force" and distinguished from inorganic materials; this separation was ended however by the synthesis of urea from inorganic precursors by Friedrich Wöhler in 1828. Other chemists who brought major contributions to organic chemistry include Alexander William Williamson with his synthesis of ethers and Christopher Kelk Ingold, among many discoveries, established the mechanisms of substitution reactions. Chemical equations are used to graphically illustrate chemical reactions, they consist of chemical or structural formulas of the reactants on the left and those of the products on the right.
They are separated by an arrow which indicates the type of the reaction.