A chemical compound is a substance created when two or more different elements are connected by chemical bonds or valence forces. Compounds and elements are both distinct substances which is one of the two basic categories of matter. The other category includes mixtures of substances.
Compounds have several defining characteristics. First, they must consist of different elements. Combinations of the same atoms form molecules, not compounds. For example, when two atoms of oxygen combine they form a molecule of oxygen, not a compound. Therefore, while every compound is a molecule, not every molecule is a compound. A compound is created through a chemical reaction that creates electrostatic bonds between its atoms. Second, when elements combine they do so in definite, unchanging ratios. A water molecule, for example, always consists of two hydrogen atoms and one oxygen atom. The proportions of individual elements in a compound are defined by the principal of stoichiometry, which is the set of rules that which governs chemical bonding. Third, a compound has different physical and chemical properties than its parent atoms. When elements are combined, they lose their individual properties. For instance, the highly reactive metallic element sodium and the poisonous gas chlorine form a crystalline compound, sodium chloride, also known as common table salt. Unlike its parent atoms, sodium chloride can be safely ingested. In addition, compounds can not be broken apart by physical or mechanical means; a chemical reaction is required to sever the chemical bonds which hold the compound together.
A chemical compound is one of the two general categories of matter. The other category includes mixtures of substances. In contrast to compounds, mixtures do not have fixed proportions. They contain different substances in varying ratios, and in mixtures, each substance retains its original properties. Furthermore, mixtures can be prepared and separated mechanically; no chemical reaction is necessary for their formation.
The composition of chemical compounds can be expressed using standard chemical symbols and notation. These represent the structures of the reactants and the resulting compounds. The three dimensional structure of compounds is described using a notation that is designated by the rules of a special branch of chemistry known as stereochemistry. It is also important to note that the same elements can form more than one compound depending on their structural arrangement. For example, three carbon atoms, four hydrogen atoms, and one oxygen atom can bond together in different formations to create either propanol (which has an hydroxyl group (OH-) bonded to one of the end carbons) or isopropanol (where the hydroxyl group is connected to the middle carbon.) When more than one compound can be formed from the same atoms, the compounds are called isomers of one another. Isomers have different chemical and physical properties even though they are composed of identical atoms.
A compound is named according to its constituent atoms. Simple compounds consisting of only two atoms are named by combining the names of both elements and changing the ending of the second element's name to "ide." For example, the compound made from silver and chlorine is called silver chloride. For more complex compounds, numbers are used to indicate how many atoms are involved. For example, carbon dioxide (CO2) has two oxygen atoms attached to one carbon atom, and phosphorous trioxide (P 22O3) has three oxygen atoms connected to two phosphorous atoms.
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