Atoms are among the most basic building blocks, making up all matter. The word atom derives from the Greek word atomos, which means “that which cannot be split.” The existence of atoms, or a fundamental, indivisible unit of matter, was proposed long before modern chemistry and physics came about. It turns out that atoms are actually made up of even smaller particles, but the atom is the smallest unit of matter that defines an element. The smaller particles that make up an atom are positively charged protons, charge-neutral neutrons, and negatively charged particles called electrons.