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Molar Mass
In chemistry , the MOLAR MASS _M_ is a physical property defined as the mass of a given substance (chemical element or chemical compound ) divided by the amount of substance . The base SI unit
SI unit
for molar mass is kg /mol . However, for historical reasons, molar masses are almost always expressed in g/mol. As an example, the molar mass of water: _M_(H2O) ≈ 6998180000000000000♠18 g/mol
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Si Unit
The INTERNATIONAL SYSTEM OF UNITS (abbreviated as SI, from the French _Système internationale (d'unités)_) is the modern form of the metric system , and is the most widely used system of measurement . It comprises a coherent system of units of measurement built on seven base units . The system also establishes a set of twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units. The system was published in 1960 as a result of an initiative that began in 1948. It is based on the metre-kilogram-second system of units (MKS) rather than any variant of the centimetre–gram–second system (CGS). SI is intended to be an evolving system, so prefixes and units are created and unit definitions are modified through international agreement as the technology of measurement progresses and the precision of measurements improves
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Chemistry
CHEMISTRY is a branch of physical science that studies the composition, structure, properties and change of matter . Chemistry includes topics such as the properties of individual atoms , how atoms form chemical bonds to create chemical compounds , the interactions of substances through intermolecular forces that give matter its general properties, and the interactions between substances through chemical reactions to form different substances. Chemistry is sometimes called the central science because it bridges other natural sciences , including physics , geology and biology . For the differences between chemistry and physics see comparison of chemistry and physics . The history of chemistry can be traced to alchemy , which had been practiced for several millennia in various parts of the world
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Mass
In physics , MASS is a property of a physical body . It is the measure of an object's resistance to acceleration (a change in its state of motion ) when a net force is applied. It also determines the strength of its mutual gravitational attraction to other bodies. The basic SI unit of mass is the kilogram (kg). Mass is not the same as weight , even though mass is often determined by measuring the object's weight using a spring scale , rather than comparing it directly with known masses . An object on the Moon would weigh less than it does on Earth because of the lower gravity, but it would still have the same mass. This is because weight is a force, while mass is the property that (along with gravity) determines the strength of this force. In Newtonian physics , mass can be generalized as the amount of matter in an object
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Chemical Element
A CHEMICAL ELEMENT or ELEMENT is a species of atoms having the same number of protons in their atomic nuclei (i.e. the same atomic number , or _Z_). There are 118 elements that have been identified, of which the first 94 occur naturally on Earth
Earth
with the remaining 24 being synthetic elements . There are 80 elements that have at least one stable isotope and 38 that have exclusively radioactive isotopes , which decay over time into other elements. Iron
Iron
is the most abundant element (by mass ) making up Earth, while oxygen is the most common element in the Earth\'s crust . Chemical elements constitute all of the ordinary matter of the universe. However astronomical observations suggest that ordinary observable matter makes up only about 15% of the matter in the universe: the remainder is dark matter ; the composition of this is unknown, but it is not composed of chemical elements
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Chemical Compound
A CHEMICAL COMPOUND (or just COMPOUND if used in the context of chemistry ) is an entity consisting of two or more atoms , at least two from different chemical elements , which associate via chemical bonds . There are four types of compounds, depending on how the constituent atoms are held together: molecules held together by covalent bonds , ionic compounds held together by ionic bonds , intermetallic compounds held together by metallic bonds , and certain complexes held together by coordinate covalent bonds . Many chemical compounds have a unique numerical identifier assigned by the Chemical Abstracts Service (CAS): its CAS number . A chemical formula is a way of expressing information about the proportions of atoms that constitute a particular chemical compound, using the standard abbreviations for the chemical elements, and subscripts to indicate the number of atoms involved
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Amount Of Substance
AMOUNT OF SUBSTANCE is a standards-defined quantity that measures the size of an ensemble of elementary entities, such as atoms, molecules, electrons, and other particles. It is sometimes referred to as CHEMICAL AMOUNT. The International System of Units
International System of Units
(SI) defines the amount of substance to be proportional to the number of elementary entities present. The SI unit
SI unit
for amount of substance is the mole . It has the unit symbol mol. The proportionality constant is the inverse of the Avogadro constant
Avogadro constant
. The mole is defined as the amount of substance that contains an equal number of elementary entities as there are atoms in 12g of the isotope carbon-12 . This number is called Avogadro's number and has the value 7023602214085700000♠6.022140857(74)×1023
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Kilogram
The KILOGRAM or KILOGRAMME ( SI unit symbol: KG) is the base unit of mass in the International System of Units (SI) (the Metric system ) and is defined as being equal to the mass of the _International Prototype of the Kilogram_ (IPK, also known as "Le Grand K" or "Big K"). The avoirdupois (or _international_) pound , used in both the imperial and US customary systems, is defined as exactly 6999453592370000000♠0.45359237 kg, making one kilogram approximately equal to 2.2046 avoirdupois pounds. Other traditional units of weight and mass around the world are also defined in terms of the kilogram, making the IPK the primary standard for virtually all units of mass on Earth
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Mole (unit)
The MOLE is the unit of measurement for amount of substance in the International System of Units (SI). The unit is defined as the amount or sample of a chemical substance that contains as many elementary entities, e.g., atoms , molecules , ions , electrons , or photons , as there are atoms in 12 grams of carbon-12 (12C), the isotope of carbon with standard atomic weight 12 by definition. This number is expressed by the Avogadro constant , which has a value of approximately 7023602214085700000♠6.022140857×1023 mol−1. The mole is an SI base unit , with the unit symbol MOL. The mole is widely used in chemistry as a convenient way to express amounts of reactants and products of chemical reactions. For example, the chemical equation 2 H2 + O2 → 2 H2O implies that 2 mol dihydrogen (H2) and 1 mol dioxygen (O2) react to form 2 mol water (H2O)
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Relative Atomic Mass
RELATIVE ATOMIC MASS (symbol: Ar) is a dimensionless (number only) physical quantity . In its modern definition, it is the ratio of the average mass of atoms of an element in a given sample to one unified atomic mass unit . The unified atomic mass unit, symbol u, is defined being  1⁄12 of the mass of a carbon-12 atom. The mass of atoms can vary (between atoms of the same element), due to the presence of various isotopes of that element. Since both values in the ratio are expressed in the same unit (u), the resulting value is dimensionless; hence the value is relative. Within one source (sample), it is a straight average over the individual atom weights (isotopes) present. Between sources, the atomic weight can vary when the source's origin (radioactive history) resulted in different isotopic concentrations. These differences are real and measurable, and can be used to identify a sample to its origin
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Standard Atomic Weight
The STANDARD ATOMIC WEIGHT (Ar, standard) or ATOMIC WEIGHT is a physical quantity for a chemical element, expressed as a relative atomic mass (Ar). It is specified by (restricted to) the IUPAC
IUPAC
(CIAAW ) definition of natural, stable, terrestrial sources. Because of this practical definition, the value is widely used as 'the' atomic weight for real life substances. For example, in pharmaceuticals and scientific research. Out of the 118 known chemical elements, 84 are stable and have this Earth-environment based value. Typically, such a value is, for example helium: Ar, standard(He) = 7000400260200000000♠4.002602(2). The "(2)" indicates the uncertainty in the last digit shown, to read 7000400260200000000♠4.002602 ±6994200000000000000♠0.000002. IUPAC also publishes abridged values, rounded to five significant figures. For helium, Ar, abridged(He) = 7000400260000000000♠4.0026
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Atom
Atom, from the Greek word atomos, which means indivisible, was first conceived around 2,400 years ago by a Greek man named Democritus . An ATOM is the smallest constituent unit of ordinary matter that has the properties of a chemical element . Every solid , liquid , gas , and plasma is composed of neutral or ionized atoms. Atoms are very small; typical sizes are around 100 picometers (a ten-billionth of a meter, in the short scale ). Atoms are small enough that attempting to predict their behavior using classical physics – as if they were billiard balls, for example – gives noticeably incorrect predictions due to quantum effects . Through the development of physics, atomic models have incorporated quantum principles to better explain and predict the behavior. Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and typically a similar number of neutrons . Protons and neutrons are called nucleons
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Molar Mass Constant
The MOLAR MASS CONSTANT, symbol Mu, is a physical constant which relates relative atomic mass and molar mass . Its value is defined to be 1 g/mol in SI units. The molar mass constant is important in writing dimensionally correct equations. It is common to see phrases such as The molar mass of an element is the atomic weight in grams per mole. However, atomic weight, i.e., relative atomic mass, is a dimensionless quantity, and cannot take the units of grams per mole. Formally, the operation is the multiplication by a constant which has the value 1 g/mol, that is the molar mass constant. The molar mass constant is unusual (but not unique) among physical constants by having an exactly defined value rather than being measured experimentally. It is fixed by the definitions of the mole and of relative atomic mass . From the definition of the mole, the molar mass of carbon 12 is exactly 12 g/mol
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Dimension
In physics and mathematics , the DIMENSION of a mathematical space (or object) is informally defined as the minimum number of coordinates needed to specify any point within it. Thus a line has a dimension of one because only one coordinate is needed to specify a point on it – for example, the point at 5 on a number line. A surface such as a plane or the surface of a cylinder or sphere has a dimension of two because two coordinates are needed to specify a point on it – for example, both a latitude and longitude are required to locate a point on the surface of a sphere. The inside of a cube , a cylinder or a sphere is three-dimensional because three coordinates are needed to locate a point within these spaces. In classical mechanics , space and time are different categories and refer to absolute space and time . That conception of the world is a four-dimensional space but not the one that was found necessary to describe electromagnetism
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Molecule
A MOLECULE is an electrically neutral group of two or more atoms held together by chemical bonds . Molecules are distinguished from ions by their lack of electrical charge . However, in quantum physics , organic chemistry , and biochemistry , the term _molecule_ is often used less strictly, also being applied to polyatomic ions . In the kinetic theory of gases , the term _molecule_ is often used for any gaseous particle regardless of its composition. According to this definition, noble gas atoms are considered molecules as they are in fact monoatomic molecules. A molecule may be homonuclear , that is, it consists of atoms of one chemical element , as with oxygen (O2); or it may be heteronuclear , a chemical compound composed of more than one element, as with water (H2O). Atoms and complexes connected by non-covalent interactions , such as hydrogen bonds or ionic bonds , are generally not considered single molecules
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Hydrogen
HYDROGEN is a chemical element with symbol H and atomic number 1. With a standard atomic weight of circa 7000100800000000000♠1.008, hydrogen is the lightest element on the periodic table . Its monatomic form (H) is the most abundant chemical substance in the Universe , constituting roughly 75% of all baryonic mass. Non-remnant stars are mainly composed of hydrogen in the plasma state . The most common isotope of hydrogen, termed _protium_ (name rarely used, symbol 1H), has one proton and no neutrons . The universal emergence of atomic hydrogen first occurred during the recombination epoch . At standard temperature and pressure , hydrogen is a colorless , odorless , tasteless , non-toxic, nonmetallic , highly combustible diatomic gas with the molecular formula H2. Since hydrogen readily forms covalent compounds with most nonmetallic elements, most of the hydrogen on Earth exists in molecular forms such as water or organic compounds
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