Here's why:

* Atoms are the building blocks: Everything around us, from the air we breathe to the chair you're sitting on, is made up of atoms.

* Indivisible (mostly): Atoms are considered the smallest unit of an element that retains the chemical properties of that element. They can't be broken down into simpler substances by ordinary chemical means. (Though, they can be broken down in nuclear reactions)

* Combined to form molecules: Atoms can combine with other atoms to form molecules, which are the smallest unit of a compound.

So, while molecules and compounds are important, the foundation of all matter is the atom.