Atomic nucleus, stability

It has recently been suggested that the imino hydrogen atom in 1,2,4-triazoles is not attached to any of the nitrogen atoms but rather that it exists as a charged atom closely bound by a negatively charged triazole nucleus stabilized by resonance (e.g., 34 and 35), but such a representation is considered to be incorrect and misleading by the present authors. 

The Structural Basis of the Magic Numbers.—Elsasser10 in 1933 pointed out that certain numbers of neutrons or protons in an atomic nucleus confer increased stability on it. These numbers, called magic numbers, played an important part in the development of the shell model 4 s it was found possible to associate them with configurations involving a spin-orbit subsubshell, but not with any reasonable combination of shells and subshells alone. The shell-model level sequence in its usual form,11 however, leads to many numbers at which subsubshells are completed, and provides no explanation of the selection of a few of them (6 of 25 in the range 0-170) as magic numbers. 

The stability of the atomic nucleus depends upon a critical balance between the repulsive and attractive forces involving the protons and neutrons. For the lighter elements, a neutron to proton ratio (N P) of about 1 1 is required for the nucleus to be stable but with increasing atomic mass, the N P ratio for a stable nucleus rises to a value of approximately 1.5 1. A nucleus whose N P ratio differs significantly from these values will undergo a nuclear reaction in order to restore the ratio and the element is said to be radioactive. There is, however, a maximum size above which any nucleus is unstable and most elements with atomic numbers greater than 82 are radioactive. 

The second reason the stabilizing effect of neutrons is limited is that any proton in the nucleus is attracted by the strong nuclear force only to adjacent protons but is electrically repelled by all other protons in the nucleus. As more and more protons are squeezed into the nucleus, the repulsive electric forces increase substantially. For example, each of the two protons in a helium nucleus feels the repulsive effect of the other. Each proton in a nucleus containing 84 protons, however, feels the repulsive effects of 83 protons The attractive nuclear force exerted by each neutron, however, extends only to its immediate neighbors. The size of the atomic nucleus is therefore limited. This in turn limits the number of possible elements in the periodic table. It is for this reason that all nuclei having more than 83 protons are radioactive. Also, the nuclei of the heaviest elements produced in the laboratory are so unstable (radioactive) that they exist for only fractions of a second. 

Synthesis of the three observations led to Bohr s proposal of a planetary atom consisting of a heavy small stationary heavy nucleus and a number of orbiting electrons. Each electron, like a planet, had its own stable orbit centred at the atomic nucleus. The simplest atom, that of hydrogen, with atomic number 1 could therefore be described as a single electron orbiting a proton at a fixed, relatively large, distance. The mechanical requirement to stabilize the orbit is a balance between electrostatic and mechanical forces, expressed in simple electrostatic units, and particle momentum p = mv, as ... 

Every atomic nucleus is lighter than the sum of the masses of the nucleons from which it is built, and this mass loss corresponds to the binding energy of the nucleus. The relative stability of two nuclei with different numbers of nucleons can be assessed by comparing their mass loss per nucleon. 

Mass spectrometry is based on the physical properties of the atomic nucleus. The atomic nucleus of any chemical element consists of protons and neutrons. In an electrically neutral atom the number of positively charged protons in the nucleus equals the number of negatively charged electrons in the shells. The number of protons (Z = atomic number) determines the chemical properties and the place of the element in the periodic table of the elements. The atomic number Z of a chemical element is given as a subscript preceding the elemental symbol (e.g., jH, gC, 17CI, 2eF or 92 )-Besides the protons, uncharged neutrons with nearly the same mass in comparison to the protons (m = 1.67493 x 10 kg versus nip = 1.67262 x 10 kg) stabilize the positive atomic nucleus. In contrast to the mass of the protons and neutrons in the nucleus, the mass of the electrons is relatively small at = 9.10939 x 10 kg. 

I his lack of chemical reactivity is the result of an extraordinary stability of the electronic structure of the helium atom. T his stability is characteristic of the presence of two electrons close to an atomic nucleus. 

The elements He, Ne, Ar, Kr, Xe, and Rn are called the argonons (or the inert gases, or the noble gases). Their atoms have little tendency to form chemical bonds. The small chemical reactivity of the argonons is attributed to the special stability of groups of 2. 10, 18. 36. 54, and 86 electrons about one atomic nucleus. 

An important feature to note is that a 3p orbital has a maximum closer to the nucleus (i.e., to the origin of the plot). That indicates that 3p electrons can approach the atomic nucleus closer than 3d electrons. As a consequence, 3p electrons are better stabilized by a positive nuclear charge and thus have lower energy than 3d electrons. 

Considering only the atomic number, one finds that of the 83 elements, 43 have even Z, and 40 odd Z (note that Tc and Pm with only short-lived isotopes have odd atomic numbers, but Th and U have even ones), which reflects the higher stability of an atomic nucleus with even number of protons. This extends further to nuclei that also have an even number of neutrons. The proton and neutron numbers in the nuclei of the 266 stable nuclides lead to the following groupings ... 

The valence orbital (2s and 2p) energy levels of the fluorine atom are stabilized remarkably well by a large positive charge of the nucleus and the absence of shielding eifects by inner-shell electrons. The orbital energy level of the 2p lies at —18.6 eV. This is 5 eV lower than that of the proton s ls-orbital [6]. 

Nuclear chemistry is very much in the news today. In addition TO APPLICATIONS IN THE MANUFACTURE OF ATOMIC BOMBS, HYDROGEN BOMBS, AND NEUTRON BOMBS, EVEN THE PE.A.CEFUL USE OF NUCLEAR ENERGY HAS BECOME CONTROVERSIAL, IN PART BECAUSE OF SAFETY CONCERNS ABOUT NUCLEAR POWER PLANTS AND ALSO BECAUSE OF PROBLEMS WITH DISPOSAL OF RADIOACTIVE WASTES. IN THIS CHAPTER WE WILL STUDY NUCLEAR REACTIONS, THE STABILITY OF THE ATOMIC NUCLEUS, RADIOACTIVITY, AND THE EFFECTS OF RADIATION ON BIOLOGICAL SYSTEMS. 

The nucleus occupies a very small portion of the total volume of an atom, but it contains most of the atom s mass because both the protons and the neutrons reside there. In studying the stability of the atomic nucleus, it is helpful to know something about its density, because it tells us how tightly the particles are packed together. As a sample calculation, let us assume that a nucleus has a radius of 5 X 10 pm and a mass of 1 X 10 g. These figures correspond roughly to a nucleus containing 30 protons and 30 neutrons. Density is mass/volume, and we can calculate the volume from the known radius (the volume of a sphere is where r is the radius of the sphere). First we convert the pm units to cm. Then we calculate the density in g/cm ... 

The atomic nucleus is a tiny massive entity at the center of an atom. Occupying a volume whose radius is 1/100,000 the size of the atom, the nucleus contains most (99.9%) of the mass of the atom. In describing the nucleus, we shall describe its composition, size, density, and the forces that hold it together. After describing the structure of the nucleus, we shall go on to describe some of the limits of nuclear stability. 

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