The term for the smallest unit of an element that retains its properties is called an atom. Atoms consist of a nucleus containing protons and neutrons, surrounded by electrons in various energy levels. Each element on the periodic table is defined by the number of protons in its nucleus, or atomic number, and distinguishes itself based on its unique arrangements and properties. Understanding atomic structure is foundational in fields like chemistry and physics, as it explains the behavior of elements and the formation of compounds and materials.