Mass defect negative

This graph and Fig. 26-6 are almost the inverse of one another, with the maxima of one being the minima of the other. Actual nuclidic mass is often a number slightly less than the number of nucleons (mass number). This difference divided by the number of nucleons (packing fraction) is proportional to the negative of the mass defect per nucleon. 

Note Commonly, the term mass defect, defined as the difference between the exact mass and the integer mass, is used to describe this deviation. [3] Application of this concept leads to positive and negative mass defects, respectively. In addition, the association of something being defective with certain isotopic masses can be misleading. 

MASS DEFECT. The difference A between the atomic number A and the atomic mass M of a nuclide. A = A - M. The negative of the mass defect. —A. is known as the mass excess. 

The relationship of energy and mass would indicate that in the formation of deuterium by the combination of a proton and neutron, the mass defect of 0.002 388 u would be observed as the liberation of an equivalent amount of energy, i.e. 931.5 X 0.002 388 = 2.224 MeV. Indeed, the emission of this amount of energy (in the form of y-rays) is observed when a proton captures a low ergy neutron to form jH. As a matter of fact, in this particular case, the energy liberated in the formation of deuterium has been used in the reverse calculation to obtain the mass of the neutron since it is not possible to determine directly the mass of the free neutron. With the definition (3.2) all stable nuclei are found to have negative AAf values thus the term "defect". 

Accurate mass scales (or calibration curves) are generally established by measuring the mass spectrum of a reference compound simultaneously with the spectrum of the sample. The precise mass of every ion in the spectrum of the reference compound is known, so a precise mass correlation is thereby provided. Common reference materials are perfluorokerosene (PFK) and perfluorotributylamine (PFTBA), the mass spectra of which are shown in Figures 3.1 and 3.2, respectively. Since all the ions formed from these compounds contain several fluorine atoms (18.9984) and no hydrogen atoms (1.0078), they have negative mass defects and are well separated from organic ions that normally have positive mass defects. Of course, other chemicals may be used to provide reference masses, as long as the exact masses in its spectrum are known. 

A reference compound is needed to determine the molecular mass. The mass of an unknown is computed by comparing its signal on the mass axis with that of the known mass reference peaks. A reference compound is also required to calibrate the data system and to tune and performance-check the instrument. A calibration standard has the following desirable characteristics (1) it should yield a sufficient number of regularly spaced abundant ions across the entire scan range (2) those reference ions should have negative mass defects to prevent overlap with the nsnal compounds containing C, H, N, and O and (3) it should be readily available, chemically inert, and sufficiently volatile. 

Fluorine has an atomic number of 9 and a relative atomic weight of 18.9984 u. This negative mass defect leads to substantially lower monoisotopic masses of highly fluorinated compounds than the respective nominal mass. For instance, the miz ratio of the perfluorooctanoate anion is 412.9664. Other organic compounds usually have monoisotopic masses higher than the respective nominal mass, since most other elements have a positive mass defect. This difference can be taken advantage of by high-resolution MS. 

Example The NICI mass spectrum of tetraiodoethene, I2C=CI2, has been obtained using isobutane reagent gas (Fig. 7.12). The negative molecular ion, NT, at m/z 531.6 has a relative intensity of just 0.15%, while the product of nucleophilic addition, [M+I]", tn/z 658.5, yields the base peak [77], Losses of T and I2 from M"" are also observed. The series of peaks at m/z 126.9, 253.8, and 380.7 corresponds to traces of iodine present as impurity of tetraiodoethene. The iodine is also ionized by both electron capture (EC, next paragraph) and iodide addition. The spectrum nicely exemplifies the superimposition of mass spectra of two components of a mixture. It is not always simple to tell the corresponding peaks apart accurate mass measurements or tandem mass spectrometry may be required. It is worth noting the mass defect introduced by the iodine and the C isotopic peak of merely 2% due to only two carbon atoms present. 

Negative U defects pin the Fermi energy. Since the upper le level is empty, and the lower 2e level is filled, must obviously lie between these two energy levels. The pinning of the Fermi energy position is demonstrated by assuming that the AT defects contain a variable density of electrons, n, where 0 < < IN. The law of mass action (see Section... 

Atomic defects, since they require for their formation the motion of particles of considerable mass, are produced most effectively by heavy particulate radiations, fast neutrons, a particles, etc. Fast electrons are considerably less effective, and electromagnetic radiations even less so. The latter can produce displacement through the recoil imparted by ejected electrons and perhaps by electrostatic repulsion following multiple ionization of a negative ion leaving it as a positive ion surrounded by positive ions (25). 

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