Radiation Chemical properties are determined by electron distributions and are only indirectly influenced by atomic nuclei . While nuclear reactions involve changes in the composition of nuclei . These extraordinary processes are often accompanied by the release of tremendous amounts of energy and by transmutations of elements. Some differences between nuclear reactions and ordinary chemical reactions are mentioned below. THE NUCLEUS The neutrons and protons together constitute the nucleus, with the electrons occupying essentially
empty space around the nucleus. The nucleus is only a minute fraction of the total volume of an atom, yet nearly all the mass of an atom resides in the nucleus. Thus, nuclei are extremely dense. It has been shown experimentally that nuclei of all elements have approximately the same density, 2.4 × 10 14 g/cm3. From an electrostatic point of view, it is amazing that positively charged protons can be packed so closely together. Yet many nuclei do not spontaneously decompose, so they must be stable. NEUTRON–PROTON RATIO AND NUCLEAR STABILITY The term “nuclide” is used to refer to different atomic forms of all elements. The term “isotope” applies only to different atomic forms of the same element. Most naturally occurring nuclides have even numbers of protons and even numbers of neutrons; 157 nuclides fall into this category. Nuclides with odd numbers of both protons and neutrons are least common (there are only four), and those with odd–even combinations are intermediate in abundance
Furthermore, nuclides with certain “magic numbers” of protons and neutrons seem to be especially stable. Nuclides with a number of protons or a number of neutrons or a sum of the two equal to 2, 8, 20, 28, 50, 82, or 126 have unusual stability. Examples are 4 2 He, 16 8 O, 42 20 Ca, 88 38 Sr, and 208 82 Pb. This suggests an energy level (shell) model for the nucleus similar to the shell model of electron configurations.
The plot above represents the number of neutrons ( N ) versus number of protons ( Z ) for the stable nuclides (the band of stability ). For low atomic numbers, the most stable nuclides have equal numbers of protons and neutrons ( N = Z ). Above atomic number 20, the most stable nuclides have more neutrons than protons. The nuclide symbol for an element is where E is the chemical symbol for the element, Z is its atomic number, and A is its mass number.
NUCLEAR STABILITY AND BINDING ENERGY Experimentally, we observe that the masses of atoms other than 1 1 H are always less than the sum of the masses of their constituent particles. We now know why this mass deficiency occurs. We also know that the mass deficiency is in the nucleus of the atom and has nothing to do with the electrons. The mass deficiency, ∆ m, for a nucleus is the difference between the sum of the masses of electrons, protons, and neutrons in the atom (calculated mass) and the actual measured mass of the atom.