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A chemical compound is a chemical substance composed of many identical (or molecular entity) containing from more than one chemical element held together by . A molecule consisting of atoms of only one element is therefore not a compound. A compound can be transformed into a different substance by a chemical reaction, which may involve interactions with other substances. In this process, bonds between atoms may be broken or new bonds formed or both.
There are four major types of compounds, distinguished by how the constituent atoms are bonded together. Molecular compounds are held together by ; are held together by ; intermetallic compounds are held together by ; coordination complexes are held together by coordinate covalent bonds. Non-stoichiometric compounds form a disputed marginal case.
A chemical formula specifies the number of atoms of each element in a compound molecule, using the standard with numerical . Many chemical compounds have a unique CAS number identifier assigned by the Chemical Abstracts Service. Globally, more than 350,000 chemical compounds (including mixtures of chemicals) have been registered for production and use.
Chemical compounds have a unique and defined chemical structure held together in a defined spatial arrangement by . Chemical compounds can be molecule compounds held together by , salts held together by , intermetallic compounds held together by , or the subset of chemical complexes that are held together by coordinate covalent bonds. Pure are generally not considered chemical compounds, failing the two or more atom requirement, though they often consist of molecules composed of multiple atoms (such as in the diatomic molecule H2, or the polyatomic molecule S8, etc.). Many chemistry compounds have a unique numerical identifier assigned by the Chemical Abstracts Service (CAS): its CAS number.
There is varying and sometimes inconsistent nomenclature differentiating substances, which include truly non-stoichiometric examples, from chemical compounds, which require the fixed ratios. Many solid chemical substances—for example many —are chemical substances, but do not have simple formulae reflecting chemical bonding of elements to one another in fixed ratios; even so, these crystalline substances are often called "non-stoichiometric compounds". It may be argued that they are related to, rather than being chemical compounds, insofar as the variability in their compositions is often due to either the presence of foreign elements trapped within the crystal structure of an otherwise known true chemical compound, or due to perturbations in structure relative to the known compound that arise because of an excess or deficit of the constituent elements at places in its structure; such non-stoichiometric substances form most of the crust and mantle of the Earth. Other compounds regarded as chemically identical may have varying amounts of heavy or light of the constituent elements, which changes the ratio of elements by mass slightly.
Ionic compounds containing basic ions like hydroxide (OH−) or oxide (O2−) are classified as bases. Ionic compounds without these ions are also known as salts and can be formed by acid–base reactions. Ionic compounds can also be produced from their constituent ions by evaporation of their solvent, precipitation, freezing, a solid-state reaction, or the electron transfer reaction of reactive metals with reactive non-metals, such as halogen gases.
Ionic compounds typically have high Melting point and , and are Hardness and Brittleness. As solids they are almost always electrically insulating, but when melting or dissolved they become highly conductive, because the ions are mobilized.
London dispersion forces, part of the Van der Waals force, are the weakest force of all intermolecular forces. They are temporary attractive forces that form when the electrons in two adjacent atoms are positioned so that they create a temporary dipole. London dispersion forces can create Van der Waals molecules. Additionally, they are responsible for condensing nonpolar substances to liquids, and to further freeze to a solid state dependent on how low the temperature of the environment is.
A covalent bond, also known as a molecular bond, involves the sharing of electrons between two atoms. Primarily, this type of bond occurs between elements that fall close to each other on the periodic table of elements, yet it is observed between some metals and nonmetals. This is due to the mechanism of this type of bond. Elements that fall close to each other on the periodic table tend to have similar electronegativities, which means they have a similar affinity for electrons. Since neither element has a stronger affinity to donate or gain electrons, it causes the elements to share electrons so both elements have a more stable octet rule.
Ionic bonding occurs when valence electrons are completely transferred between elements. Opposite to covalent bonding, this chemical bond creates two oppositely charged ions. The metals in ionic bonding usually lose their valence electrons, becoming a positively charged cation. The nonmetal will gain the electrons from the metal, making the nonmetal a negatively charged anion. As outlined, ionic bonds occur between an electron donor, usually a metal, and an electron acceptor, which tends to be a nonmetal.
Hydrogen bonding occurs when a hydrogen atom bonded to an electronegative atom forms an electrostatic connection with another electronegative atom through interacting dipoles or charges.
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