"A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) composed of atoms from more than one element held together by chemical bonds. A molecule consisting of atoms of only one element is therefore not a compound.
There are four types of compounds, depending on how the constituent atoms are held together:
molecules held together by covalent bonds
ionic compounds held together by ionic bonds
intermetallic compounds held together by metallic bonds
certain complexes held together by coordinate covalent bonds.
A chemical formula specifies the number of atoms of each element in a compound molecule, using the standard abbreviations for the chemical elements and numerical subscripts. For example, a water molecule has formula H2O indicating two hydrogen atoms bonded to one oxygen atom. 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." - (en.wikipedia.org 13.08.2021)
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