The smallest unit of an element that retains its properties is an atom. Atoms are composed of a nucleus containing protons and neutrons, surrounded by electrons. Each element on the periodic table consists of its unique kind of atom, characterized by the number of protons in the nucleus, known as the atomic number. Atoms can combine to form molecules, and their interactions underlie all chemical reactions. Understanding atomic structure is fundamental in chemistry and contributes to advancements in various scientific fields.