Stable nucleus

The nucleus is not involved in normal chemical reactions. In radioactivity, it is the nucleus that is undergoes the changes. Nuclear forces are extremely strong forces that hold the nuclear particles together. These forces make the nucleus very stable. Remember that the protons are positively charged and can exert repulsive forces among themselves. It has been found out that there are certain predictable behavioral patterns in elements as the atomic number increases. As the atomic number increases, the neutron-to-proton ratio increases and as more and more protons are present in the nucleus, the stabilizing forces are not sufficient to keep the nucleus stable. Thus the stability of the nucleus decreases as the neutron-to-proton ratio increases. 

ISeed nucleus Stable nucleus Unstable nucleus... 

Radioactivity is believed to be a result of an unstable ratio of neutrons to protons in the nucleus. Stable nuclides of elements up to about atomic number 20 generally have about a 1 1 neutron-to-proton ratio. In elements above number 20, the neu-tron-to-proton ratio in the stable nuclides gradually increases to about 1.5 1 in element number 83 (bismuth). When the neutron-to-proton ratio is too high or too low, alpha, beta, or other particles are emitted to achieve a more stable nucleus. 

Light-Induced Alkylation and Dealkylation of the Flavin Nucleus. Stable... 

All elements of atomic number greater than 83 exhibit radioactive decay K, Rb, Ir and a few other light elements emit p particles. The heavy elements decay through various isotopes until a stable nucleus is reached. Known half-lives range from seconds to 10 years. 

Element 103, lawrencium, completes the actinides. Following this series, the transition elements should continue with the filling of the 6d orbitals. There is evidence for an element 104 (eka-hafnium) it is believed to form a chloride MCl4, similar to that of hafnium. Less positive evidence exists for elements 105 and 106 attempts (so far unsuccessful) have been made to synthesise element 114 (eka-lead). because on theoretical grounds the nucleus of this elemeni may be stable to decay by spontaneous fusion (as indeed is lead). Super-... 

Radioactivity occurs naturally in earth minerals containing uranium and thorium. It also results from two principal processes arising from bombardment of atomic nuclei by particles such as neutrons, ie, activation and fission. Activation involves the absorption of a neutron by a stable nucleus to form an unstable nucleus. An example is the neutron reaction of a neutron and cobalt-59 to yield cobalt-60 [10198 0-0] Co, a 5.26-yr half-life gamma-ray emitter. Another is the absorption of a neutron by uranium-238 [24678-82-8] to produce plutonium-239 [15117 8-5], Pu, as occurs in the fuel of a nuclear... 

The compounds of this article, ie, ftve-membered heterocycles containing two adjacent nitrogen atoms, can best be discussed according to the number of double bonds present. Pyrazoles contain two double bonds within the nucleus, imparting an aromatic character to these molecules. They are stable compounds and can display the isomeric forms, (1) and (2), when properly substituted. Pyrazoles are scarce ia nature when compared to the imidazoles (3), which are widespread and have a central role ia many biological processes. 

A radioisotope is an atom the nucleus of which is not stable and which decays to a more stable state by the emission of various radiations. Radioactive isotopes, also called nucHdes or radionucHdes, are important to many areas of scientific research, as well as ia medical and iadustrial appHcations (see... 

A free neutron has a half-life of 10.4 minutes, but a neutron is stable when bound in a nucleus. 

In order to obtain a homogenous and stable latex compound, it is necessary that insoluble additives be reduced in particle size to an optimum of ca 5 )Tm and dispersed or emulsified in water. Larger-size chemical particles form a nucleus for agglomeration of smaller particles and cause localized dispersion instabiHty particles <3 fim tend to cluster with similar effect, and over-milled zinc oxide dispersions are particularly prone to this. Water-soluble ingredients, including some accelerators, can be added directly to the latex but should be made at dilute strength and at similar pH value to that of the latex concentrate. 

The reactivity of the amino groups at the pteridine nucleus depends very much upon their position. All amino groups form part of amidine or guanidine systems and therefore do not behave like benzenoid amino functions which can usually be diazotized. The 4-, 6-and 7-amino groups are in general subject to hydrolysis by acid and alkali, whereas the 2-amino group is more stable under these conditions but is often more susceptible to removal by nitrous acid. 

When hydroxypteridines are considered, it must be borne in mind that these compounds exist principally in the pteridinone forms, containing thermodynamically stable amide functions, and consequently have low reactivity. Their stability towards acid and alkali correlates well with the number of electron-donating groups which apparently redress the deficit of ir-electrons located at the ring nitrogen atoms. Quantitative correlations can be seen in the decomposition studies of various pteridinones (Table 7). These results are consistent with the number of the oxy functions and their site at the pteridine nucleus. The... 

The critical size of the stable nucleus at any degree of under cooling can be calculated widr an equation derived similarly to that obtained earlier for the concentration of defects in a solid. The configurational entropy of a mixture of nuclei containing n atoms widr o atoms of the liquid per unit volume, is given by the Boltzmann equation... 

This result has been plotted out in Fig. 7.1. It shows that there is a maximum value for Wjr corresponding to a critical radius r. For r < r (dV /dr) is positive, whereas for r > r it is negative. This means that if a random fluctuation produces a nucleus of size r < r it will be unstable the system can do free work if the nucleus loses atoms and r decreases. The opposite is true when a fluctuation gives a nucleus with r > r. Then, free work is done when the nucleus gains atoms, and it will tend to grow. To summarise, if random fluctuations in the liquid give crystals with r > r then stable nuclei will form, and solidification can begin. 

Pure titanium is cooled from a temperature at which the b.c.c. phase is stable to a temperature at which the c.p.h. phase is stable. As a result, lens-shaped nuclei of the c.p.h. phase form at the grain boundaries. Estimate the number of atoms needed to make a critical-sized nucleus given the following data AH = 3.48 kJ moT atomic weight = 47.90 - T = 30 K = 882°C y= 0.1 ]ra density of the c.p.h. 

Nucleation is the growth of clusters of molecules that become a thermodynamically stable nucleus. This process is dependent on the vapor pressure of the condensable species. The molecular clusters undergo growth when the saturation ratio, S, is greater than 1, where saturation ratio is defined as the actual pressure of the gas divided by its equilibrium vapor pressure. S > 1 is referred to as a supersaturated condition (14). 

Decay The spontaneous disintegration of an unstable atomic nucleus to form another more stable element or isotope of a lower atomic mass. 

This length is apparently related to the capture time by the relation Pi J Tc and il A physical meaning of the free diffusion length 4 is that the maximum size of a stable adsorbed two-dimensional nucleus on a facet cannot essentially exceed this free diffusion length. If the nucleus is smaller, all atoms depositing on the surface can still find the path to the boundary of a nucleus in order to be incorporated there. If the nucleus is larger, a new nucleus can develop on its surface. 

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