What is an atom? Where is it used?

Atoms are the basic units of matter and the defining structure of elements. Atoms are made up of three particles: protons, neutrons and electrons. Protons and neutrons are heavier than electrons and reside in the center of the atom, which is called the nucleus.

An atom a fundamental piece of matter. (Matter is anything that can be touched physically). Everything in the universe (except energy) is made of matter, and, so, everything in the universe is made of atoms.