All matter in the universe is made out of atoms. An atom is the smallest particle of an element that has the properties of that element. Atoms contain three types of subatomic particles: protons, neutrons, and electrons, with the exception of hydrogen, whose one isotope does not contain any neutrons. A proton is a positively charged subatomic particle in the nucleus of an atom. A neutron is a subatomic particle that has a neutral charge in the nucleus of an atom. The mass of the atom is concentrated at the atomic nucleus, which is a positively charged center of an atom containing protons and neutrons. The mass of one proton is equal to the mass of one neutron, or 1 atomic mass unit (amu). An electron is a negatively charged subatomic particle. Electrons move freely around the nucleus of the atom and contribute a negligible, but existent, amount of mass to the overall mass of the atom. Since electrons are in constant motion around the nucleus, they can move from one atom to another atom. The negative charge of one electron is exactly opposite and equal to the positive charge of one proton.An element is a substance composed of atoms that have the same number of protons. The number of protons in each atom of an element is called the atomic number. There are 118 known elements, but only 92 of them are found on Earth naturally; the remaining 26 elements have been produced in laboratories and are unstable. Each element has a unique atomic number and is identified by a one- or two-letter chemical symbol. The element with the atomic number 2 is helium, which consists of 2 protons and has the chemical symbol He. Carbon has 6 protons, an atomic number of 6, and the chemical symbol C. Sodium has 11 protons, an atomic number of 11, and the chemical symbol Na.