A molecule is a unit of matter consisting of two or more atoms combined in a definite ratio. When the atoms are the same in a molecule the substance is an element. When the atoms are different the substance is considered a compound. Another way of defining a compound is a pure substance containing more than one kind of atom.

What would a molecule of of an element look like? Since it would consist of the same atoms a molecule with two atoms could be represented as .

But a molecule can also have different atoms. In such a case the molecule would be the simplest particle of the compound that retains the properties of the compound. What would a molecule of a compound look like? A molecule which contained three atoms, two of one type and one of another might look like or . (The order of the atoms and the geometry of the arrangement is something we will discuss later in the semester.)