The fundamental building block of all matter is the atom. Atoms consist of a nucleus, containing protons and neutrons, surrounded by electrons that orbit the nucleus. Atoms combine to form molecules, which are the basis for all chemical compounds and substances in the universe. The properties and interactions of atoms determine the characteristics of matter, influencing its behavior in various states (solid, liquid, gas). Understanding atoms is essential in fields like chemistry, physics, and materials science, as they underpin the structure and function of all materials.