A group of atoms joined together is called a molecule. Molecules are the fundamental building blocks of matter and can consist of two or more atoms that are chemically bonded together.
For instance, a water molecule (H2O) is formed by two hydrogen atoms and one oxygen atom. The way these atoms bond determines the properties of the molecule. Molecules can be simple, like oxygen gas (O2), or complex, like DNA, which consists of thousands of atoms connected to perform biological functions.