An atom is a fundamental piece of matter that is the basic building block of all matter in the universe. It is the smallest unit of matter that retains the properties of an element and cannot be divided into smaller particles without releasing electrically charged particles. Atoms consist of three types of subatomic particles: protons, neutrons, and electrons. The nucleus of an atom is made up of protons and neutrons, which are relatively heavy particles, while electrons are negatively charged particles that fly around the nucleus in a small cloud. The number of protons in the nucleus determines the atomic number of the element, which in turn determines where that atom fits on the periodic table and the characteristics of the element. Atoms are roughly the same size, whether they have 3 or 90 electrons, and are measured in angstroms. The physicist Ernest Rutherford developed an early model of the atom in 1912, which was later revised by Niels Bohr in 1913.