Is Water a Compound or an Element?

Water is everywhere on our planet and it's the reason we have organic life. It shapes our mountains, carves our oceans, and drives our weather. It would be logical to think that water must be one of the basic elements. But in reality, water is a chemical compound.

Water as a Compound and Molecule

A compound forms whenever two or more atoms form chemical bonds with each other. The chemical formula for water is H₂O, which means each molecule of water consists of one oxygen atom chemically bonded to two hydrogen atoms. Thus, water is a compound. It's also a molecule, which is any chemical species formed by two or more atoms chemically bonded to each other. The terms "molecule" and "compound" mean the same thing and can be used interchangeably.

Sometimes confusion arises because the definitions of molecule and compound haven't always been so clear-cut. In the past, some schools taught that molecules consisted of atoms bonded via covalent chemical bonds, while compounds were formed via ionic bonds. The hydrogen and oxygen atoms in water are covalently bonded, so under these older definitions, water would be a molecule but not a compound. An example of a compound would be table salt, NaCl. However, as scientists came to understand chemical bonding better, the line between ionic and covalent bonds became fuzzier. Also, some molecules contain both ionic and covalent bonds between the various atoms.

In summary, the modern definition of a compound is a type of molecule consisting of at least two different types of atoms. By this definition, water is both a molecule and a compound. Oxygen gas (O₂) and ozone (O₃), for example, are substances that are molecules but not compounds.

Why Water Is Not an Element

Before mankind knew about atoms and molecules, water was considered an element. Other elements included earth, air, fire, and sometimes metal, wood, or spirit. In some traditional sense, you could consider water an element, but it doesn't qualify as an element according to the scientific definition—an element is a substance consisting of only one type of atom. Water consists of two types of atoms: hydrogen and oxygen.

How Water Is Unique

Though water is everywhere on Earth, it is a very unusual compound because of the nature of the chemical bonds between its atoms. Here are a few of its eccentricities:

• Water is denser in its liquid state than in its solid state, which is why ice can float on or in liquid water.
• Water has an unusually high boiling point based on its molecular weight.
• Water is often referred to as the "universal solvent" because of its amazing ability to dissolve so many substances.

These unusual properties have had a profound impact on the development of life on Earth and on the weathering and erosion of Earth's surface. Other planets that are not water-rich have had very different natural histories.