The smallest unit of an element is the atom. Atoms are the basic building blocks of matter and consist of a nucleus containing protons and neutrons, surrounded by a cloud of electrons. Each element on the periodic table is defined by its unique type of atom, characterized by the number of protons in its nucleus, known as the atomic number. Atoms can bond together to form molecules, which make up all substances in the universe, showcasing the fundamental nature of atoms in chemistry and physics.