Overview
Chemical elements are often found within chemical compounds, in which one or more elements are bonded together. Some elements exist freely in compounds with one another, while others are bonded by electrostatic charges and still others are bonded with molecular bonds.
Molecules
A molecule is the smallest uncharged unit of a compound, containing at least two atoms. Unlike elements, molecules can always be divided into individual atoms by chemical changes. For example, the smallest molecule of water (H2O) contains 2 atoms of hydrogen to every atom of oxygen, in exactly that proportion. Molecules of water can be frozen to form solid ice or heated to form gaseous water vapor, but those physical changes do not change its chemical composition. However, if water molecules react chemically with another substance, then new molecules of other substances are formed.
Diatomic Molecules
Some chemical elements almost always exist freely in molecules of two atoms of the element. For example, free hydrogen exists as H2, free oxygen as O2, and free nitrogen as N2. Other elements that exist freely as diatomic molecules include fluorine, F2, a pale yellow gas; chlorine, Cl2, a yellow-green gas; bromine, Br2, a reddish-brown liquid; and iodine, I2, a bluish-black solid. However, when any of those elements form other compounds, they are no longer free. For example, water contains both hydrogen and oxygen, but neither element is free, just two atoms of hydrogen combined with one atom of oxygen in a water molecule.
Ionic Compounds
The atoms in ionic compounds are bound together by the attraction between negatively charged ions, called anions, and positively charged ions, called cations. Common salt, sodium chloride (NaCl), consists of positively charged sodium cations and negatively charged chlorine anions that form the compound. Ionic compounds tend to be soluble, and many conduct electricity when they are in solution.
Molecular Compounds
Molecules such as water are bound together by shared electrons between elements rather than by electrostatic charges. Bonds may be between the electrons of two or more different atoms, to form many different types of structures.
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