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Introduction
Published in Armen S. Casparian, Gergely Sirokman, Ann O. Omollo, Rapid Review of Chemistry for the Life Sciences and Engineering, 2021
Armen S. Casparian, Gergely Sirokman, Ann O. Omollo
The mass number, A, of an element is the sum of the number of protons and the number of neutrons in the nucleus, collectively known as the number of nucleons. Protons and neutrons are assigned a mass number of one atomic mass unit (amu) each, based on the carbon-12 atom as the standard. Although an element can have only one atomic number, it may have more than one mass number. This fact gives rise to the phenomenon of isotopes. Isotopes are atoms of the same element with the same atomic number but with a different number of neutrons. An element may have several isotopes, one or more of which may be radioactive and hence unstable. For example, oxygen has three naturally occurring isotopes, all of which are stable, while carbon also has three, one of which is radioactive. The mathematical average of all isotopic mass numbers of an element, weighted by the percent abundance in nature of these isotopes, constitutes the element’s atomic mass. It is impossible to predict theoretically how many isotopes an element may have or how many may be radioactive. The isotopes of a given element have almost identical chemical properties (i.e., reactivity) but different physical properties (i.e., density, melting point, etc.). The mass number is expressed as a left-hand superscript to the element symbol, for example, 12C.
Area Monitoring and Contingency Planning
Published in Martha J. Boss, Dennis W. Day, Air Sampling and Industrial Hygiene Engineering, 2020
The atomic number of an atom designates its specific elemental identity. For example, an atom with a Z = 1 is hydrogen, an atom with Z = 2 is helium, while Z = 3 identifies an atom of lithium. Atoms characterized by a particular atomic number and atomic mass are called nuclides. A specific nuclide is represented by its chemical symbol with the atomic mass in a superscript (e.g., 3H, 14C, 125I). Nuclides with the same number of protons (i.e., same Z) but different number of neutrons (i.e., different A) are called isotopes. Isotopes of a particular element have nearly identical chemical properties.
Analytical Methods
Published in S. Komar Kawatra, Advanced Coal Preparation and Beyond, 2020
Mössbauer spectroscopy differs from the other spectrographic techniques discussed, in that it uses the absorption of γ-rays by the nucleus of the target atom, rather than the absorption by the electron shells. The atomic nuclei are the heavy cores of atoms and are generally considered to be composed of protons and neutrons. The number of protons in the nucleus is the atomic number of the atom, and this value determines the chemical properties of the atom. The Mössbauer effect is concerned with the emission of γ-rays by a radioactive nucleus and the subsequent reabsorption of these γ-rays by other nuclei of the same type as the emitting atom, as shown in Figure 2.1. For this absorption to occur, the γ-ray must match the resonance of the absorbing atom very closely, and so the technique is extremely selective.
Elastic scattering of e∓ by Na atoms
Published in Molecular Physics, 2018
M. Elias Hosain, M. Atiqur R. Patoary, M. M. Haque, A. K. Fazlul Haque, M. Ismail Hossain, M. Alfaz Uddin, Arun K. Basak, M. Maaza, Bidhan C. Saha
The components of the real part Vst(r), Vex(r), Vcp(r) in (15) are the static, exchange and correlation–polarisation potentials, respectively. The imaginary component Wabs(r) represents an absorptive part which incorporates the absorption of particles into various inelastic channels during the collisions. The static potential Vst(r) has been generated from the procedure adopted earlier by Salvat et al. [52], where the proton and electron densities are, respectively, accounted for by means of Fermi nuclear charge distribution [54] and multi-configuration Dirac–Fock programme of Desclaux [55]. For electron impact Na scattering, we have used the semi-classical exchange potential of Furness and McCarthy [56] derived directly from the formal expression of the non-local exchange interaction by using a Wentzel, Kramers, and Brillouin (WKB)-like approximation for the wave functions. This potential can be expressed as Here, Ei is the impact energy of electron, a0 is the Bohr radius, ρ(r) is the electron density function normalised as with Z being the atomic number of the target atom.