An atom is the smallest unit of matter that defines a chemical element. It is composed of a central nucleus made up of protons and neutrons, which is surrounded by a cloud of negatively charged electrons. The number of protons in the nucleus, known as the atomic number, determines the element to which the atom belongs. Atoms are the basic building blocks of matter, and anything that has mass and occupies space is composed of atoms. The basic idea that matter is made up of tiny indivisible particles is an old idea that appeared in many ancient cultures. The word atom is derived from the ancient Greek word atomos, which means "uncuttable". Modern atomic theory is not based on these old concepts. The physicist Ernest Rutherford developed an early model of the atom in 1912. The study of atoms and their behavior is called atomic physics.