Ammonium chloride is an inorganic compound with the formula NH4Cl and a white crystalline salt, soluble in water. Solutions of ammonium chloride are mildly acidic. Sal ammoniac is a name of the mineralogical form of ammonium chloride; the mineral is formed on burning coal dumps from condensation of coal-derived gases. It is found around some types of volcanic vents, it is used as fertilizer and a flavouring agent in some types of liquorice. It is the product from the reaction of hydrochloric ammonia, it is a product of the Solvay process used to produce sodium carbonate: CO2 + 2 NH3 + 2 NaCl + H2O → 2 NH4Cl + Na2CO3In addition to being the principal method for the manufacture of ammonium chloride, that method is used to minimize ammonia release in some industrial operations. Ammonium chloride is prepared commercially by combining ammonia with either hydrogen chloride or hydrochloric acid: NH3 + HCl → NH4ClAmmonium chloride occurs in volcanic regions, forming on volcanic rocks near fume-releasing vents.
The crystals deposit directly from the gaseous state and tend to be short-lived, as they dissolve in water. Ammonium chloride appears to sublime upon heating but decomposes into ammonia and hydrogen chloride gas. NH4Cl → NH3 + HClAmmonium chloride reacts with a strong base, like sodium hydroxide, to release ammonia gas: NH4Cl + NaOH → NH3 + NaCl + H2OSimilarly, ammonium chloride reacts with alkali metal carbonates at elevated temperatures, giving ammonia and alkali metal chloride: 2 NH4Cl + Na2CO3 → 2 NaCl + CO2 + H2O + 2 NH3A 5% by weight solution of ammonium chloride in water has a pH in the range 4.6 to 6.0. Some of ammonium chloride's reactions with other chemicals are endothermic like its reaction with barium hydroxide and its dissolving in water; the dominant application of ammonium chloride is as a nitrogen source in fertilizers such as chloroammonium phosphate. The main crops fertilized this way are wheat in Asia. Ammonium chloride was used in pyrotechnics in the 18th century but was superseded by safer and less hygroscopic chemicals.
Its purpose was to provide a chlorine donor to enhance the green and blue colours from copper ions in the flame. It had a secondary use to provide white smoke, but its ready double decomposition reaction with potassium chlorate producing the unstable ammonium chlorate made its use suspect. Ammonium chloride galvanized or soldered, it works as a flux by cleaning the surface of workpieces by reacting with the metal oxides at the surface to form a volatile metal chloride. For that purpose, it is sold in blocks at hardware stores for use in cleaning the tip of a soldering iron, it can be included in solder as flux. Ammonium chloride is used as an expectorant in cough medicine, its expectorant action is caused by irritative action on the bronchial mucosa, which causes the production of excess respiratory tract fluid, easier to cough up. Ammonium salts may induce nausea and vomiting. Ammonium chloride is used as a systemic acidifying agent in treatment of severe metabolic alkalosis, in oral acid loading test to diagnose distal renal tubular acidosis, to maintain the urine at an acid pH in the treatment of some urinary-tract disorders.
Ammonium chloride, under the name sal ammoniac or salmiak is used as food additive under the E number E510, working as a yeast nutrient in breadmaking and as an acidifier. It is a feed supplement for cattle and an ingredient in nutritive media for yeasts and many microorganisms. Ammonium chloride is used to spice up dark sweets called salmiak, in baking to give cookies a crisp texture, in the liquor Salmiakki Koskenkorva for flavouring. In Iran, India and Arab countries it is called "Noshader" and is used to improve the crispness of snacks such as samosas and jalebi. Ammonium chloride has been used to produce low temperatures in cooling baths. Ammonium chloride solutions with ammonia are used as buffer solutions including ACK lysis buffer. In paleontology, ammonium chloride vapor is deposited on fossils, where the substance forms a brilliant white removed and harmless and inert layer of tiny crystals; that covers up any coloration the fossil may have, if lighted at an angle enhances contrast in photographic documentation of three-dimensional specimens.
The same technique is applied in archaeology to eliminate reflection on glass and similar specimens for photography. In organic synthesis saturated NH4Cl solution is used to quench reaction mixtures. Giant squid and some other large squid species maintain neutral buoyancy in seawater through an ammonium chloride solution, found throughout their bodies and is less dense than seawater; this differs from the method of flotation used by most fish, which involves a gas-filled swim bladder. The solution tastes somewhat like salmiakki and makes giant squid unattractive for general human consumption. Ammonium chloride is used in a ~5% aqueous solution to work on oil wells with clay swelling problems, it is used as electrolyte in zinc–carbon batteries. Other uses include in hair shampoo, in the glue that bonds plywood, in cleaning products. In hair shampoo, it is used as a thickening agent in ammonium-based surfactant systems such as ammonium lauryl sulfate. Ammonium chloride is used in the textile and leather industry, in dyeing, textile printing and cotton clustering.
Around the turn of the 20th Century, Ammonium Chloride was used in aqueous solution as the electrolyte in
In chemistry, pH is a scale used to specify how acidic or basic a water-based solution is. Acidic solutions have a lower pH, while basic solutions have a higher pH. At room temperature, pure water is neither acidic nor basic and has a pH of 7; the pH scale is logarithmic and approximates the negative of the base 10 logarithm of the molar concentration of hydrogen ions in a solution. More it is the negative of the base 10 logarithm of the activity of the hydrogen ion. At 25 °C, solutions with a pH less than 7 are acidic and solutions with a pH greater than 7 are basic; the neutral value of the pH depends on the temperature, being lower than 7 if the temperature increases. Contrary to popular belief, the pH value can be less than 0 or greater than 14 for strong acids and bases respectively; the pH scale is traceable to a set of standard solutions whose pH is established by international agreement. Primary pH standard values are determined using a concentration cell with transference, by measuring the potential difference between a hydrogen electrode and a standard electrode such as the silver chloride electrode.
The pH of aqueous solutions can be measured with a glass electrode and a pH meter, or a color-changing indicator. Measurements of pH are important in chemistry, medicine, water treatment, many other applications; the concept of pH was first introduced by the Danish chemist Søren Peder Lauritz Sørensen at the Carlsberg Laboratory in 1909 and revised to the modern pH in 1924 to accommodate definitions and measurements in terms of electrochemical cells. In the first papers, the notation had the "H" as a subscript to the lowercase "p", as so: pH; the exact meaning of the "p" in "pH" is disputed, but according to the Carlsberg Foundation, pH stands for "power of hydrogen". It has been suggested that the "p" stands for the German Potenz, others refer to French puissance. Another suggestion is that the "p" stands for the Latin terms pondus hydrogenii, potentia hydrogenii, or potential hydrogen, it is suggested that Sørensen used the letters "p" and "q" to label the test solution and the reference solution.
In chemistry, the p stands for "decimal cologarithm of", is used in the term pKa, used for acid dissociation constants. Bacteriologist Alice C. Evans, famed for her work's influence on dairying and food safety, credited William Mansfield Clark and colleagues with developing pH measuring methods in the 1910s, which had a wide influence on laboratory and industrial use thereafter. In her memoir, she does not mention how much, or how little and colleagues knew about Sørensen's work a few years prior, she said: In these studies Dr. Clark's attention was directed to the effect of acid on the growth of bacteria, he found that it is the intensity of the acid in terms of hydrogen-ion concentration that affects their growth. But existing methods of measuring acidity determined not the intensity, of the acid. Next, with his collaborators, Dr. Clark developed accurate methods for measuring hydrogen-ion concentration; these methods replaced the inaccurate titration method of determining acid content in use in biologic laboratories throughout the world.
They were found to be applicable in many industrial and other processes in which they came into wide usage. The first electronic method for measuring pH was invented by Arnold Orville Beckman, a professor at California Institute of Technology in 1934, it was in response to local citrus grower Sunkist that wanted a better method for testing the pH of lemons they were picking from their nearby orchards. PH is defined as the decimal logarithm of the reciprocal of the hydrogen ion activity, aH+, in a solution. PH = − log 10 = log 10 For example, for a solution with a hydrogen ion activity of 5×10−6 we get 1/ = 2×105, thus such a solution has a pH of log10 = 5.3. For a commonplace example based on the facts that the masses of a mole of water, a mole of hydrogen ions, a mole of hydroxide ions are 18 g, 1 g, 17 g, a quantity of 107 moles of pure water, or 180 tonnes, contains close to 1 g of dissociated hydrogen ions and 17 g of hydroxide ions. Note that pH depends on temperature. For instance at 0 °C the pH of pure water is 7.47.
At 25 °C it's 7.00, at 100 °C it's 6.14. This definition was adopted because ion-selective electrodes, which are used to measure pH, respond to activity. Ideally, electrode potential, E, follows the Nernst equation, for the hydrogen ion can be written as E = E 0 + R T F ln = E 0 − 2.303 R T F pH where E is a measured potential, E0 is the standard electrode potential, R is the gas const
An acid is a molecule or ion capable of donating a hydron, or, capable of forming a covalent bond with an electron pair. The first category of acids is the proton donors or Brønsted acids. In the special case of aqueous solutions, proton donors form the hydronium ion H3O+ and are known as Arrhenius acids. Brønsted and Lowry generalized the Arrhenius theory to include non-aqueous solvents. A Brønsted or Arrhenius acid contains a hydrogen atom bonded to a chemical structure, still energetically favorable after loss of H+. Aqueous Arrhenius acids have characteristic properties which provide a practical description of an acid. Acids form aqueous solutions with a sour taste, can turn blue litmus red, react with bases and certain metals to form salts; the word acid is derived from the Latin acidus/acēre meaning sour. An aqueous solution of an acid has a pH less than 7 and is colloquially referred to as'acid', while the strict definition refers only to the solute. A lower pH means a higher acidity, thus a higher concentration of positive hydrogen ions in the solution.
Chemicals or substances having the property of an acid are said to be acidic. Common aqueous acids include hydrochloric acid, acetic acid, sulfuric acid, citric acid; as these examples show, acids can be solutions or pure substances, can be derived from acids that are solids, liquids, or gases. Strong acids and some concentrated weak acids are corrosive, but there are exceptions such as carboranes and boric acid; the second category of acids are Lewis acids. An example is boron trifluoride, whose boron atom has a vacant orbital which can form a covalent bond by sharing a lone pair of electrons on an atom in a base, for example the nitrogen atom in ammonia. Lewis considered this as a generalization of the Brønsted definition, so that an acid is a chemical species that accepts electron pairs either directly or by releasing protons into the solution, which accept electron pairs. However, hydrogen chloride, acetic acid, most other Brønsted-Lowry acids cannot form a covalent bond with an electron pair and are therefore not Lewis acids.
Conversely, many Lewis acids are not Brønsted-Lowry acids. In modern terminology, an acid is implicitly a Brønsted acid and not a Lewis acid, since chemists always refer to a Lewis acid explicitly as a Lewis acid. Modern definitions are concerned with the fundamental chemical reactions common to all acids. Most acids encountered in everyday life are aqueous solutions, or can be dissolved in water, so the Arrhenius and Brønsted-Lowry definitions are the most relevant; the Brønsted-Lowry definition is the most used definition. Hydronium ions are acids according to all three definitions. Although alcohols and amines can be Brønsted-Lowry acids, they can function as Lewis bases due to the lone pairs of electrons on their oxygen and nitrogen atoms; the Swedish chemist Svante Arrhenius attributed the properties of acidity to hydrogen ions or protons in 1884. An Arrhenius acid is a substance that, when added to water, increases the concentration of H+ ions in the water. Note that chemists write H+ and refer to the hydrogen ion when describing acid-base reactions but the free hydrogen nucleus, a proton, does not exist alone in water, it exists as the hydronium ion, H3O+.
Thus, an Arrhenius acid can be described as a substance that increases the concentration of hydronium ions when added to water. Examples include molecular substances such as acetic acid. An Arrhenius base, on the other hand, is a substance which increases the concentration of hydroxide ions when dissolved in water; this decreases the concentration of hydronium because the ions react to form H2O molecules: H3O+ + OH− ⇌ H2O + H2ODue to this equilibrium, any increase in the concentration of hydronium is accompanied by a decrease in the concentration of hydroxide. Thus, an Arrhenius acid could be said to be one that decreases hydroxide concentration, while an Arrhenius base increases it. In an acidic solution, the concentration of hydronium ions is greater than 10−7 moles per liter. Since pH is defined as the negative logarithm of the concentration of hydronium ions, acidic solutions thus have a pH of less than 7. While the Arrhenius concept is useful for describing many reactions, it is quite limited in its scope.
In 1923 chemists Johannes Nicolaus Brønsted and Thomas Martin Lowry independently recognized that acid-base reactions involve the transfer of a proton. A Brønsted-Lowry acid is a species. Brønsted-Lowry acid-base theory has several advantages over Arrhenius theory. Consider the following reactions of acetic acid, the organic acid that gives vinegar its characteristic taste: CH3COOH + H2O ⇌ CH3COO− + H3O+ CH3COOH + NH3 ⇌ CH3COO− + NH+4Both theories describe the first reaction: CH3COOH acts as an Arrhenius acid because it acts as a source of H3O+ when dissolved in water, it acts as a Brønsted acid by donating a proton to water. In the second example CH3COOH undergoes the same transformation, in this case donating a proton to ammonia, but does not relate to the Arrhenius definition of an acid because the reaction does not produce hydronium. CH3COOH is
Calcium chloride is an inorganic compound, a salt with the chemical formula CaCl2. It is a white coloured crystalline solid at room temperature soluble in water. Calcium chloride is encountered as a hydrated solid with generic formula CaCl2x, where x = 0, 1, 2, 4, 6; these compounds are used for de-icing and dust control. Because the anhydrous salt is hygroscopic, it is used as a desiccant. By depressing the freezing point of water, calcium chloride is used to prevent ice formation and is used to de-ice; this application consumes the greatest amount of calcium chloride. Calcium chloride is harmless to plants and soil; as a de-icing agent, it is much more effective at lower temperatures than sodium chloride. When distributed for this use, it takes the form of small, white spheres a few millimeters in diameter, called prills. Solutions of calcium chloride can prevent freezing at temperature as low as −52 °C, making it ideal for filling agricultural implement tires as a liquid ballast, aiding traction in cold climates.
It is used in domestic and industrial chemical air dehumidifiers. The second largest application of calcium chloride exploits hygroscopic properties and the tackiness of its hydrates. A concentrated solution keeps a liquid layer on the surface of dirt roads, which suppresses formation of dust, it keeps. If these are allowed to blow away, the large aggregate begins to shift around and the road breaks down. Using calcium chloride reduces the need for grading by as much as 50% and the need for fill-in materials as much as 80%; the average intake of calcium chloride as food additives has been estimated to be 160–345 mg/day. Calcium chloride is permitted as a food additive in the European Union for use as a sequestrant and firming agent with the E number E509, it is considered as recognized as safe by the U. S. Food and Drug Administration, its use in organic crop production is prohibited under the US National Organic Program. In marine aquariums, calcium chloride is one way to introduce bioavailable calcium for calcium carbonate-shelled animals such as mollusks and some cnidarians.
Calcium hydroxide or a calcium reactor can be used. As a firming agent, calcium chloride is used in canned vegetables, in firming soybean curds into tofu and in producing a caviar substitute from vegetable or fruit juices, it is used as an electrolyte in sports drinks and other beverages, including bottled water. The salty taste of calcium chloride is used to flavor pickles without increasing the food's sodium content. Calcium chloride's freezing-point depression properties are used to slow the freezing of the caramel in caramel-filled chocolate bars, it is added to sliced apples to maintain texture. In brewing beer, calcium chloride is sometimes used to correct mineral deficiencies in the brewing water, it affects flavor and chemical reactions during the brewing process, can affect yeast function during fermentation. In cheesemaking, calcium chloride is sometimes added to processed milk to restore the natural balance between calcium and protein in casein, it is added before the coagulant. Calcium chloride is used to prevent cork spot and bitter pit on apples by spraying on the tree during the late growing season.
Drying tubes are packed with calcium chloride. Kelp is dried with calcium chloride for use in producing sodium carbonate. Anhydrous calcium chloride has been approved by the FDA as a packaging aid to ensure dryness; the hydrated salt can be dried for re-use but will dissolve in its own water of hydration if heated and form a hard amalgamated solid when cooled. Calcium chloride is used in concrete mixes to accelerate the initial setting, but chloride ions lead to corrosion of steel rebar, so it should not be used in reinforced concrete; the anhydrous form of calcium chloride may be used for this purpose and can provide a measure of the moisture in concrete. Calcium chloride is included as an additive in plastics and in fire extinguishers, in wastewater treatment as a drainage aid, in blast furnaces as an additive to control scaffolding, in fabric softener as a thinner; the exothermic dissolution of calcium chloride is used in self-heating cans and heating pads. In the oil industry, calcium chloride is used to increase the density of solids-free brines.
It is used to provide inhibition of swelling clays in the water phase of invert emulsion drilling fluids. CaCl2 acts as flux material in the Davy process for the industrial production of sodium metal, through the electrolysis of molten NaCl. CaCl2 is used as a flux and electrolyte in the FFC Cambridge process for titanium production, where it ensures the proper exchange of calcium and oxygen ions between the electrodes. Calcium chloride is used in the production of activated charcoal. Calcium chloride is an ingredient used in ceramic slipware, it suspends clay particles so that they float within the solution making it easier to use in a variety of slipcasting techniques. Calcium chloride dihydrate dissolved in ethanol has been used as a sterilant for male animals; the solution is injected into the testes of the animal. Within 1 month, necrosis of testicular tissue results in sterilization. Calcium chloride can act as an irritant by desiccating moist skin. Solid calcium chloride dissolves exothermically, burns can result in the mouth and esophagus if it is ingested.
Ingestion of concentrated solutions or solid products may cause gastrointestinal irritation or ulceration. Consumption of calcium
Urea known as carbamide, is an organic compound with chemical formula CO2. This amide has two –NH2 groups joined by a carbonyl functional group. Urea serves an important role in the metabolism of nitrogen-containing compounds by animals and is the main nitrogen-containing substance in the urine of mammals, it is a colorless, odorless solid soluble in water, non-toxic. Dissolved in water, it is neither alkaline; the body uses it in most notably nitrogen excretion. The liver forms it by combining two ammonia molecules with a carbon dioxide molecule in the urea cycle. Urea is used in fertilizers as a source of nitrogen and is an important raw material for the chemical industry. Friedrich Wöhler's discovery in 1828 that urea can be produced from inorganic starting materials was an important conceptual milestone in chemistry, it showed for the first time that a substance known only as a byproduct of life could be synthesized in the laboratory without biological starting materials thereby contradicting the held doctrine of vitalism.
More than 90% of world industrial production of urea is destined for use as a nitrogen-release fertilizer. Urea has the highest nitrogen content of all solid nitrogenous fertilizers in common use. Therefore, it has the lowest transportation costs per unit of nitrogen nutrient. Many soil bacteria possess the enzyme urease, which catalyzes conversion of urea to ammonia or ammonium ion and bicarbonate ion, thus urea fertilizers transform to the ammonium form in soils. Among the soil bacteria known to carry urease, some ammonia-oxidizing bacteria, such as species of Nitrosomonas, can assimilate the carbon dioxide the reaction releases to make biomass via the Calvin cycle, harvest energy by oxidizing ammonia to nitrite, a process termed nitrification. Nitrite-oxidizing bacteria Nitrobacter, oxidize nitrite to nitrate, mobile in soils because of its negative charge and is a major cause of water pollution from agriculture. Ammonium and nitrate are absorbed by plants, are the dominant sources of nitrogen for plant growth.
Urea is used in many multi-component solid fertilizer formulations. Urea is soluble in water and is therefore very suitable for use in fertilizer solutions, e.g. in'foliar feed' fertilizers. For fertilizer use, granules are preferred over prills because of their narrower particle size distribution, an advantage for mechanical application; the most common impurity of synthetic urea is biuret. Urea is spread at rates of between 40 and 300 kg/ha but rates vary. Smaller applications incur lower losses due to leaching. During summer, urea is spread just before or during rain to minimize losses from volatilization; because of the high nitrogen concentration in urea, it is important to achieve an spread. The application equipment must be calibrated and properly used. Drilling must not occur on contact with or close to seed, due to the risk of germination damage. Urea dissolves in water for application through irrigation systems. In grain and cotton crops, urea is applied at the time of the last cultivation before planting.
In high rainfall areas and on sandy soils and where good in-season rainfall is expected, urea can be side- or top-dressed during the growing season. Top-dressing is popular on pasture and forage crops. In cultivating sugarcane, urea is side-dressed after planting, applied to each ratoon crop. In irrigated crops, urea can be applied dry to the soil, or dissolved and applied through the irrigation water. Urea dissolves in its own weight in water, but becomes difficult to dissolve as the concentration increases. Dissolving urea in water is endothermic—the solution temperature falls when urea dissolves; as a practical guide, when preparing urea solutions for fertigation, dissolve no more than 3 g urea per 1 L water. In foliar sprays, urea concentrations of between 0.5% and 2.0% are used in horticultural crops. Low-biuret grades of urea are indicated. Urea absorbs moisture from the atmosphere and therefore is stored either in closed or sealed bags on pallets or, if stored in bulk, under cover with a tarpaulin.
As with most solid fertilizers, storage in a cool, well-ventilated area is recommended. Overdose or placing urea near seed is harmful. Urea is a raw material for the manufacture of two main classes of materials: urea-formaldehyde resins and urea-melamine-formaldehyde used in marine plywood. Urea can be used to make urea nitrate, a high explosive, used industrially and as part of some improvised explosive devices, it is a stabilizer in nitrocellulose explosives. Urea is used in SNCR and SCR reactions to reduce the NOx pollutants in exhaust gases from combustion from Diesel, dual fuel, lean-burn natural gas engines; the BlueTec system, for example, injects a water-based urea solution into the exhaust system. The ammonia produced by the hydrolysis of the urea reacts with the nitrogen oxide emissions and is converted into nitrogen and water within the catalytic converter. Trucks and cars using these catalytic converters need to carry a supply of diesel exhaust fluid, a solution of urea in water. Urea in concentrations up to 10 M is a powerful protein denaturant as it disrupts the noncovalent bonds in the proteins.
This property can be exploited to increase the solubility of some proteins. A mixture of urea and choline chloride is used as
Mammals are vertebrate animals constituting the class Mammalia, characterized by the presence of mammary glands which in females produce milk for feeding their young, a neocortex, fur or hair, three middle ear bones. These characteristics distinguish them from reptiles and birds, from which they diverged in the late Triassic, 201–227 million years ago. There are around 5,450 species of mammals; the largest orders are the rodents and Soricomorpha. The next three are the Primates, the Cetartiodactyla, the Carnivora. In cladistics, which reflect evolution, mammals are classified as endothermic amniotes, they are the only living Synapsida. The early synapsid mammalian ancestors were sphenacodont pelycosaurs, a group that produced the non-mammalian Dimetrodon. At the end of the Carboniferous period around 300 million years ago, this group diverged from the sauropsid line that led to today's reptiles and birds; the line following the stem group Sphenacodontia split off several diverse groups of non-mammalian synapsids—sometimes referred to as mammal-like reptiles—before giving rise to the proto-mammals in the early Mesozoic era.
The modern mammalian orders arose in the Paleogene and Neogene periods of the Cenozoic era, after the extinction of non-avian dinosaurs, have been among the dominant terrestrial animal groups from 66 million years ago to the present. The basic body type is quadruped, most mammals use their four extremities for terrestrial locomotion. Mammals range in size from the 30–40 mm bumblebee bat to the 30-meter blue whale—the largest animal on the planet. Maximum lifespan varies from two years for the shrew to 211 years for the bowhead whale. All modern mammals give birth to live young, except the five species of monotremes, which are egg-laying mammals; the most species-rich group of mammals, the cohort called placentals, have a placenta, which enables the feeding of the fetus during gestation. Most mammals are intelligent, with some possessing large brains, self-awareness, tool use. Mammals can communicate and vocalize in several different ways, including the production of ultrasound, scent-marking, alarm signals and echolocation.
Mammals can organize themselves into fission-fusion societies and hierarchies—but can be solitary and territorial. Most mammals are polygynous. Domestication of many types of mammals by humans played a major role in the Neolithic revolution, resulted in farming replacing hunting and gathering as the primary source of food for humans; this led to a major restructuring of human societies from nomadic to sedentary, with more co-operation among larger and larger groups, the development of the first civilizations. Domesticated mammals provided, continue to provide, power for transport and agriculture, as well as food and leather. Mammals are hunted and raced for sport, are used as model organisms in science. Mammals have been depicted in art since Palaeolithic times, appear in literature, film and religion. Decline in numbers and extinction of many mammals is driven by human poaching and habitat destruction deforestation. Mammal classification has been through several iterations since Carl Linnaeus defined the class.
No classification system is universally accepted. George Gaylord Simpson's "Principles of Classification and a Classification of Mammals" provides systematics of mammal origins and relationships that were universally taught until the end of the 20th century. Since Simpson's classification, the paleontological record has been recalibrated, the intervening years have seen much debate and progress concerning the theoretical underpinnings of systematization itself through the new concept of cladistics. Though field work made Simpson's classification outdated, it remains the closest thing to an official classification of mammals. Most mammals, including the six most species-rich orders, belong to the placental group; the three largest orders in numbers of species are Rodentia: mice, porcupines, beavers and other gnawing mammals. The next three biggest orders, depending on the biological classification scheme used, are the Primates including the apes and lemurs. According to Mammal Species of the World, 5,416 species were identified in 2006.
These were grouped into 153 families and 29 orders. In 2008, the International Union for Conservation of Nature completed a five-year Global Mammal Assessment for its IUCN Red List, which counted 5,488 species. According to a research published in the Journal of Mammalogy in 2018, the number of recognized mammal species is 6,495 species included 96 extinct; the word "mammal" is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the Latin mamma. In an influential 1988 paper, Timothy Rowe defined Mammalia phylogenetically as the crown group of mammals, the clade consisting of the most recent common ancestor of living monotremes and therian m