ATOMIC NATURE OF MATTER DOE-HDBK-1019/1-93 Atomic and Nuclear Physics Solution: The  name  of  the  element  can  be  found  from  the  Periodic  Table  (refer  to  Chemistry Fundamentals  Handbook)  or  the  Chart  of  the  Nuclides  (to  be  discussed  later).    The number  of  protons  and  electrons  are  equal  to  Z.    The  number  of  neutrons  is  equal to Z - A. Nuclide Element Protons Electrons Neutrons 1 1 H hydrogen 1 1 0 10 5 B boron 5 5 5 14 7 N nitrogen 7 7 7 11 4 4 8 Cd cadmium 48 48 66 23 9 9 4 Pu plutonium 94 94 145 Isotopes Isotopes are nuclides that have the same atomic number and are therefore the same element, but differ in the number of neutrons.  Most elements have a few stable isotopes and several unstable, radioactive isotopes.   For example, oxygen has three stable isotopes that can be found in nature (oxygen-16,  oxygen-17,  and  oxygen-18)  and  eight  radioactive  isotopes.    Another  example  is hydrogen, which has two stable isotopes (hydrogen-1 and hydrogen-2) and a single radioactive isotope (hydrogen-3). The  isotopes  of  hydrogen  are  unique  in  that  they  are  each  commonly  referred  to  by  a  unique name instead of the common chemical element name.   Hydrogen-1 is almost always referred to as  hydrogen,  but  the  term protium  is  infrequently  used also.   Hydrogen-2  is  commonly called deuterium and symbolized ²₁  D.  Hydrogen-3 is commonly called tritium and symbolized ³₁  T.  This text will normally use the symbology ²₁  H and ³₁  H for deuterium and tritium, respectively. Atomic and Nuclear Radii The size of an atom is difficult to define exactly due to the fact that the electron cloud, formed by  the  electrons  moving  in  their  various  orbitals,  does  not  have  a  distinct  outer  edge.    A reasonable   measure  of atomic  size  is  given  by the  average  distance of  the outermost  electron from  the  nucleus.     Except  for  a  few  of  the  lightest  atoms,  the  average  atomic  radii  are approximately the same for all atoms, about 2 x 10 -8 cm. Like the atom the nucleus does not have a sharp outer boundary.   Experiments have shown that the nucleus is shaped like a sphere with a radius that depends on the atomic mass number of the atom.   The relationship between the atomic mass number and the radius of the nucleus is shown in the following equation. NP-01 Page 6 Rev. 0