A chemical compound is a substance consisting of two or more different elements chemically bonded together in a fixed proportion by mass.^[1][2] Water and steam are two different forms of the same chemical substance A chemical substance is a material with a definite chemical composition. ... The periodic table of the chemical elements A chemical element, or element, is a type of atom that is distinguished by its atomic number; that is, by the number of protons in its nucleus. ... In chemistry, a chemical bond is the force which holds together atoms in molecules or crystals. ... For other uses, see Mass (disambiguation). ...

Nature of a chemical compound

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. 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. A compound therefore may not be completely homogenous, but for most purposes in chemistry it can be regarded as such. Enargite crystals In mineralogy and crystallography, a crystal structure is a unique arrangement of atoms in a crystal. ... For other uses, see Isotope (disambiguation). ... Homogeneous is an adjective that has several meanings. ...

Not all molecules are compounds. A diatomic molecule of hydrogen, represented by H₂, is homonuclear — made of atoms of only one element, so is not regarded as a compound. Compounds are pure substances that contain two or more elements combined in a definite fixed proportion. A computer rendering of the Nitrogen Molecule, which is a diatomic molecule. ... This article is about the chemistry of hydrogen. ...

Compounds compared to mixtures

Compounds have different physical and chemical properties from their constituent elements. 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. 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. Mixtures can be made by mechanical means alone. A physical property is any aspect of an object or substance that can be measured or perceived without changing its identity. ... The phrase chemical property is context-dependent, but generally refers to a materials quality which becomes evident during a chemical reaction; this is, which can only be established by changing a substances chemical identity. ... In chemistry, a mixture is a substance made by combining two or more different materials in such a way that no chemical reaction occurs. ... Combustion, an example of a chemical change. ... For other uses, see Chemical reaction (disambiguation). ...

An example of a mixture that is often confused to be a compound is an alloy. 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. An alloy is a homogeneous hybrid of two or more elements, at least one of which is a metal, and where the resulting material has metallic properties. ...

Formula

Main article: Chemical formula

Chemists describe compounds using formula in various formats. 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.g. NaCl for table salt. The order of the elements in molecular and empirical formulas is C, then H and then alphabetical. Trifluoroacetic acid is thus described as C₂HF₃O₂. 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. A more descriptive presentation would be O₂S(OH)₂. A chemical formula is an easy way of expressing information about the atoms that constitute a particular chemical compound. ... A polymer (from Greek: πολυ, polu, many; and μέρος, meros, part) is a substance composed of molecules with large molecular mass composed of repeating structural units, or monomers, connected by covalent chemical bonds. ... For other uses, see Mineral (disambiguation). ... This article is about metallic materials. ... An oxide is a chemical compound containing at least one oxygen atom and other elements. ... Edible salt is a mineral, one of the few rocks people eat. ... Trifluoroacetic acid (TFA) is a strong, hygroscopic, non-oxidizing, organic acid with a molecular formula C2HF3O2. ... Traditionally, inorganic compounds are considered to be of mineral, not biological, origin. ...

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. 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). In chemistry, valence electrons are the electrons contained in the valence shell of an atom, and which are likely to participate in a chemical reaction through bonding with other atoms or molecules. ... The noble gases are a chemical series. ... General Name, symbol, number helium, He, 2 Chemical series noble gases Group, period, block 18, 1, s Appearance colorless Standard atomic weight 4. ...

Phases and thermal properties

Compounds may have several possible phases. All compounds can exist as solids, at least at low enough temperatures. Molecular compounds may also exist as liquids, gases, and, in some cases, even plasmas. All compounds decompose upon applying heat.. The temperature at which such fragmentation occurs is often called the decomposition temperature. 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. 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 (i. ... This box:      For other uses, see Solid (disambiguation). ... It has been suggested that Molecular compound be merged into this article or section. ... For other uses, see Liquid (disambiguation). ... For other uses, see Gas (disambiguation). ... For other uses, see Plasma. ... For other uses, see Heat (disambiguation) In physics, heat, symbolized by Q, is energy transferred from one body or system to another due to a difference in temperature. ... For other uses, see Temperature (disambiguation). ... The Decomposition Temperature of a substance is the temperature at which the substance decomposes into smaller substances or into its constituent atoms. ... For other uses, see Atom (disambiguation). ...

CAS number

Every chemical compound that has been described in the literature carries a unique numerical identifier, its CAS number. This article discusses the use of the word Number in Mathematics. ... Identifiers (IDs) are lexical tokens that name entities. ... CAS registry numbers are unique numerical identifiers for chemical compounds, polymers, biological sequences and alloys. ...

References