The smallest unit of matter is known as an atom. Atoms consist of a nucleus made up of protons and neutrons, surrounded by a cloud of electrons. They are the basic building blocks of all matter and play a crucial role in the structure and properties of elements and compounds. Different combinations of atoms form molecules, which can vary widely in size and complexity. Understanding atoms is fundamental to the fields of chemistry, physics, and materials science, as they govern the interactions and behaviors of all substances in the universe.