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A chemical compound is a type of molecule (collection of bonded atoms) made up of more than one element. For instance, water (H2O) is a compound, whereas diatomic hydrogen (H2) is not. Most of the atoms that make up the Earth’s crust and living organisms are in compound form. Compounds are represented by molecular formulae which list the constituent atoms and their abundance.
Depending on the atoms, a compound may be held together by different chemical bonds. Bonds are formed through the interactions of atomic electron shells in various orbital configurations. Depending on the number of electrons orbiting the nucleus of the atom and the amount of space in the orbitals, certain atoms are prone to bonding with certain other types of atoms. Lewis dots, a graphical system of representing compound bond types, named after the chemist Gilbert Lewis, is often taught in High School chemistry classes. The existence of chemical bonds has been speculated about since at least the 12th century, although the entire field was poorly understood until the concept of electron bonds was introduced by Lewis in 1916.
When an atom possesses a “full” electron shell, it is very nonreactive and tends not to bond to other atoms. These elements can be found on the far right of the Periodic Table and are called the noble gases. Examples include helium, neon, and argon.
In chemical compounds, the most common type of bond is a covalent bond, where both constituent atoms have similar electronegativity, resulting in an even sharing of electrons. Electron orbitals overlap, and a single electron may orbit both atomic nuclei, not belonging to either exclusively. An example of a molecule held together by covalent bonds is diamond. A second type of bond, the ionic bond, occurs in atoms with significant differences in electronegativity, where that one atom dominates the supply of electrons. These tend to be slightly weaker than covalent bonds. An example is sodium chloride, or salt.
Other bond types exist: metallic bonds, van der Waals forces, and several others. Together, these make up all the chemical compounds in the universe.