Atoms are the smallest unit of an element that retains the chemical properties of that element. They are composed of:

* Protons: Positively charged particles located in the atom's nucleus.

* Neutrons: Neutral particles (no charge) also located in the nucleus.

* Electrons: Negatively charged particles that orbit the nucleus in shells or energy levels.

The number of protons in an atom determines what element it is. For example, all carbon atoms have 6 protons, while all oxygen atoms have 8 protons. The number of neutrons can vary, leading to different isotopes of the same element.

While atoms are the fundamental building blocks of matter, they themselves are made of even smaller particles called quarks and leptons. Quarks make up protons and neutrons, while leptons include electrons.

So, in summary, all matter in the universe is ultimately made up of quarks and leptons, which combine to form atoms.