Atomic Nature of Matter Summary

ATOMIC NATURE OF MATTER DOE-HDBK-1019/1-93 Atomic and Nuclear Physics Atomic Nature of Matter Summary Atoms consist of three basic subatomic particles. These particles are the proton, the neutron, and the electron. Protons are particles that have a positive charge, have about the same mass as a hydrogen atom, and exist in the nucleus of an atom. Neutrons are particles that have no electrical charge, have about the same mass as a hydrogen atom, and exist in the nucleus of an atom. Electrons are particles that have a negative charge, have a mass about eighteen hundred times smaller than the mass of a hydrogen atom, and exist in orbital shells around the nucleus of an atom. The Bohr model of the atom consists of a dense nucleus of protons and neutrons (nucleons) surrounded by electrons traveling in discrete orbits at fixed distances from the nucleus. Nuclides are atoms that contain a particular number of protons and neutrons. Isotopes are nuclides that have the same atomic number and are therefore the same element, but differ in the number of neutrons. The atomic number of an atom is the number of protons in the nucleus. The mass number of an atom is the total number of nucleons (protons and neutrons) in the nucleus. The notation ^A_Z X is used to identify a specific nuclide. "Z" represents the atomic number, which is equal to the number of protons. "A" represents the mass number, which is equal to the number of nucleons. "X" represents the chemical symbol of the element. Number of protons = Z Number of electrons = Z Number of neutrons = A - Z The stability of a nucleus is determined by the different forces interacting within it. The electrostatic force is a relatively long-range, strong, repulsive force that acts between the positively charged protons. The nuclear force is a relatively short-range attractive force between all nucleons. The gravitational force the long range, relatively weak attraction between masses, is negligible compared to the other forces. NP-01 Page 10 Rev. 0