Particles

All matter is made of particles. Some particles are atoms, and some are molecules. Some molecules are elements, and others are compounds. All atoms are elements, but some elements are molecules. Got that?! No? How about a diagram:

ATOM: the smallest unit of any ELEMENT that can exist on its own

MOLECULE: any particle with more than one ATOM bonded together

COMPOUND: any particle with more than one ELEMENT bonded together

ELEMENT: a unique type of ATOM named on the Periodic Table

Atom

The smallest part of any element is called an atom. This is one atom of the element argon, which you can identify by counting the number of electrons – 18.

Argon is a noble gas, so exists as single atoms, a.k.a. monoatomic gas.

Molecular element

Most elements do not exist as single atoms, as most elements have electrons available for exchange.

Oxygen is an element that exists as a diatomic gas, i.e. two oxygen atoms bonded together, O₂.

Molecular compound

Most molecules are made of more than one element: these are called compounds.

Cellulose is a compound. Each molecule of cellulose is made of 6 carbon atoms, 10 hydrogen atoms, and 5 oxygen atoms, bonded together in this specific configuration.