Chemical compound
Substance of multiple elements bonded in fixed proportions.
A chemical compound is a substance made up of many identical molecules, each containing atoms from two or more different chemical elements linked by chemical bonds. If a molecule consists of atoms of just one element, it is not a compound. Through chemical reactions—which can break existing bonds, form new ones, or both—a compound can be changed into a different substance. Compounds fall into four main categories based on how their atoms are bonded: molecular compounds (covalent bonds), ionic compounds (ionic bonds), intermetallic compounds (metallic bonds), and coordination complexes (coordinate covalent bonds). Non-stoichiometric compounds are a debated exception. A chemical formula shows the number of atoms of each element in a compound molecule, using standard element symbols and numerical subscripts. Many compounds are assigned a unique CAS number by the Chemical Abstracts Service. Worldwide, over 350,000 chemical compounds (including mixtures) have been registered for production and use.
- types
- molecular, ionic, intermetallic, coordination complexes
- defining feature
- atoms from more than one element in fixed stoichiometric proportion
- identifier
- CAS number assigned by Chemical Abstracts Service
Lore & Background
A chemical compound is a substance composed of many identical molecules, each containing atoms from more than one chemical element, held together by chemical bonds. A molecule made of only one element is not a compound. Compounds can be transformed into different substances through chemical reactions, which may break or form bonds between atoms. The four major types are molecular compounds (covalent bonds), ionic compounds (ionic bonds), intermetallic compounds (metallic bonds), and coordination complexes (coordinate covalent bonds); non-stoichiometric compounds are a disputed marginal case. A chemical formula uses standard symbols and numerical subscripts to show the number of atoms of each element in a molecule. Many compounds have a unique CAS number identifier. Over 350,000 chemical compounds have been registered globally for production and use. The term "compound" has been used since at least 1661, when Robert Boyle’s *The Sceptical Chymist* employed terms like "compound" and "perfectly mixt body." Boyle used the concept of corpuscles, or atoms, to explain how a limited number of elements could combine into a vast number of compounds. Isaac Watts, in his 1724 *Logick*, gave an early definition of chemical element and contrasted it with compound in clear modern terms. Any substance with two or more different types of atoms in fixed stoichiometric proportions is a chemical compound; non-stoichiometric compounds have reproducible but non-integral proportions. Chemical compounds have a defined spatial arrangement of atoms held by chemical bonds.
Reader's Guide
The modern definition of a chemical compound requires a fixed stoichiometric proportion of two or more different types of atoms, held together by chemical bonds. These bonds can be covalent, ionic, metallic, or coordinate covalent, giving rise to molecular compounds, ionic compounds, intermetallic compounds, and coordination complexes, respectively. A molecule consisting of atoms of only one element is not considered a compound. Many chemical compounds have a unique CAS number assigned by the Chemical Abstracts Service, and globally over 350,000 compounds have been registered for production and use. The concept of a compound has been used since at least 1661, when Robert Boyle, in *The Sceptical Chymist*, used terms like "compound" and "compounded body" and explained how a limited number of elements could combine into many compounds through "corpuscles" or "atomes." By 1724, Isaac Watts gave an early definition of chemical element and contrasted it with compound in clear, modern terms. A chemical formula, such as H₂O for water, expresses the number of atoms of each element in a compound. Non-stoichiometric compounds, where proportions are reproducible but not integral (e.g., palladium hydride, PdHx with 0.02 < x < 0.58), are a disputed marginal case. Many solid chemical substances, like silicate minerals, do not have simple fixed-ratio formulae and are often called non-stoichiometric compounds, forming most of the Earth's crust and mantle.
Did You Know?
- A molecule consisting of atoms of only one element is not a compound.
- Non-stoichiometric compounds form a disputed marginal case.
Frequently Asked Questions
What is a Chemical Compound?
A chemical compound is a substance built from identical molecules, each containing atoms of two or more different elements locked together by chemical bonds. It represents the next level of matter beyond single-element substances.
What separates a Chemical Compound from a pure element?
The defining rule is that every molecule in a compound must include atoms of more than one element in a fixed stoichiometric ratio. A collection of atoms from only one element, however bonded, simply does not qualify as a compound.
What types of Chemical Compounds exist?
Four major families are recognized: molecular compounds (covalent bonds), ionic compounds (ionic bonds), intermetallic compounds (metallic bonds), and coordination complexes (coordinate covalent bonds). Non-stoichiometric compounds occupy a disputed marginal category outside these four.
How is a Chemical Compound uniquely identified?
Each recognized compound is assigned a CAS (Chemical Abstracts Service) number, which serves as its universal registry identifier across the chemical literature and databases.
What happens to a Chemical Compound during a chemical reaction?
A compound can be broken apart or reassembled into entirely new substances when its existing bonds are broken and new ones form. This bond-breaking-and-forming process is the core mechanism by which chemistry converts one set of compounds into another.
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