A chemical compound is a substance consisting of two or more different elements chemically bonded together in a fixed proportion by mass. A chemical substance is a Material with a definite chemical composition. A chemical element is a type of Atom that is distinguished by its Atomic number; that is by the number of Protons in its nucleus. A chemical bond is the physical process responsible for the attractive interactions between Atoms and Molecules and which confers stability to diatomic and polyatomic Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object ^[1][2]

Contents 1 Nature 1.1 Fine-tuning the definition 1.2 Compounds compared to mixtures 2 Formula 3 Phases and thermal properties 4 CAS number 5 References

Nature

Fine-tuning the definition

There are some exceptions to the definition above. Certain crystalline compounds may be treated as chemical compounds despite varying in composition according to the presence or otherwise of elements trapped within the crystal structure. In Mineralogy and Crystallography, a crystal structure is a unique arrangement of Atoms in a Crystal. Some compounds regarded as chemically identical may have varying amounts of heavy or light isotopes of the constituent elements, which will make the ratio of elements by mass vary slightly. Isotopes (Greek isos = "equal" tópos = "site place" are any of the different types of atoms ( Nuclides A compound therefore may not be completely homogenous, but for most purposes in chemistry it can be regarded as such.

Not all molecules are compounds. In Chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two Atoms in a definite arrangement held together by A diatomic molecule of hydrogen, represented by H₂, is homonuclear — made of atoms of only one element, so is not regarded as a compound. Diatomic molecules are molecules made only of two Atoms of either the same or different Chemical elements The prefix di- means two in Greek Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 Compounds are pure substances that contain two or more elements combined in a definite fixed proportion.

Compounds compared to mixtures

Compounds have different physical and chemical properties from their constituent elements. A physical property is any aspect of an object or substance that can be measured or perceived without changing its identity. A chemical property is any of a material's properties that becomes evident during a Chemical reaction; that is any quality that can be established only by changing a substance's This is the one principal criterion for distinguishing a compound from a mixture of elements or substances: a mixture's properties are generally similar or related to the properties of its constituents. In Chemistry, a mixture is a substance made by combining two or more different materials without a chemical reaction occurring (the objects do not bond together Another criterion is that the constituents of a mixture can usually be separated by simple, mechanical means such as filtering, those of a compound are often very hard to separate. Furthermore, when a compound is formed from its constituents, a chemical change takes place through chemical reactions. Is a process in which Reactants are changed into one or more different products. A chemical reaction is a process that always results in the interconversion of Chemical substances The substance or substances initially involved in a chemical reaction are called Mixtures can be made by mechanical means alone.

An example of a mixture that is often confused to be a compound is an alloy. An alloy is a Solid solution or Homogeneous mixture of two or more elements, at least one of which is a Metal, which itself has It is made mechanically, most commonly by heating up all of the constituent(s) and then cooling it quickly so that the constituents are then "caught" in the base metal.

Formula

Main article: Chemical formula

Chemists describe compounds using formula in various formats. A chemical formula is a way of expressing information about the Atoms that constitute a particular Chemical compound, and how the relationship between those atoms changes For molecules, the formula for the molecular unit is shown. For polymeric materials, such as minerals and many metal oxides, the empirical formula is given, e. A polymer is a large Molecule ( Macromolecule) composed of repeating Structural units typically connected by Covalent Chemical bonds A mineral is a naturally occurring substance formed through geological processes that has a characteristic chemical composition a highly ordered atomic structure and specific The M acro E xpansion T emplate A ttribute L anguage complements TAL, providing macros which allow the reuse of code across An oxide is a Chemical compound containing at least one Oxygen atom as well as at least one other element g. NaCl for table salt. Salt is a Dietary mineral composed primarily of Sodium chloride that is essential for Animal life but toxic to most land plants The order of the elements in molecular and empirical formulas is C, then H and then alphabetical. Trifluoroacetic acid is thus described as C₂HF₃O₂. Trifluoroacetic acid (TFA is the Chemical compound with the formula CF3CO2H More descriptive formulas convey structure information, illustrated again with trifluoroacetic acid. CF₃CO₂H. On the other hand, formulas for inorganic compounds often do not convey structural information, as illustrated by H₂SO₄ for a molecule that has no H-S bonds. Traditionally inorganic compounds are considered to be of mineral not biological origin A more descriptive presentation would be O₂S(OH)₂.

Elements form compounds to become more stable. They become stable when they have the maximum number of possible electrons in their outermost energy level, which is normally two or eight valence electrons. In chemistry valence electrons are the Electrons contained in the outermost or valence, Electron shell of an Atom. This is the reason that noble gases do not frequently react: they already possess eight valence electrons (the exception being helium, which requires only two valence electrons to achieve stability). History Noble gas is translated from the German noun de ''Edelgas'' first used in 1898 by Hugo Erdmann to indicate their extremely low level of reactivity Helium ( He) is a colorless odorless tasteless non-toxic Inert Monatomic Chemical

Phases and thermal properties

Compounds may have several possible phases. In the Physical sciences a phase is a Set of states of a macroscopic physical system that have relatively uniform chemical composition and physical properties All compounds can exist as solids, at least at low enough temperatures. A solid' object is in the States of matter characterized by resistance to Deformation and changes of Volume. Molecular compounds may also exist as liquids, gases, and, in some cases, even plasmas. In Chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two Atoms in a definite arrangement held together by Liquid is one of the principal States of matter. A liquid is a Fluid that has the particles loose and can freely form a distinct surface at the boundaries of This page is about the physical properties of gas as a state of matter In Physics and Chemistry, plasma is an Ionized Gas, in which a certain proportion of Electrons are free rather than being bound All compounds decompose upon applying heat. In Physics, heat, symbolized by Q, is Energy transferred from one body or system to another due to a difference in Temperature . The temperature at which such fragmentation occurs is often called the decomposition temperature. Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature For the biological process see Decomposition. For chemical decomposition in general see Chemical decomposition. Decomposition temperatures are not sharp and depend on the rate of heating. At sufficiently high temperatures, all compounds, either after they have decomposed somehow or in the act of decomposing, fragment into smaller compounds or to individual atoms. History See also Atomic theory, Atomism The concept that matter is composed of discrete units and cannot be divided into arbitrarily tiny

CAS number

Every chemical compound that has been described in the literature carries a unique numerical identifier, its CAS number. A number is an Abstract object, tokens of which are Symbols used in Counting and measuring. In Computer science, Identifiers ( IDs) are lexical tokens that name entities. CAS registry numbers are unique numerical identifiers for Chemical compounds Polymers biological sequences mixtures and Alloys They are also referred to

References

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