The term for the smallest unit of a chemical element is an "atom." Atoms are the fundamental building blocks of matter and consist of a nucleus made of protons and neutrons, surrounded by a cloud of electrons. Different types of atoms correspond to different elements, defined by the number of protons in their nucleus, known as the atomic number. Atoms can combine to form molecules, leading to the wide variety of substances we see in the world. Understanding atoms and their interactions is fundamental to the fields of chemistry and physics.