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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 atomic masses are given for each element in the periodic table in amus or atomic mass units. The mass number of a given isotope of an element is the sum of the number of protons and the number of neutrons in its nucleus. Recall that in Example 1.1, the mass numbers of the three isotopes of uranium were given as U-234, U-235, and U-238. However, this is not equivalent to the atomic weight. Most elements have more than one isotope, so the natural distribution of the isotopes of an element must also be taken into consideration. Using the carbon-12 isotope as the standard for mass, atomic masses can then be assigned to all the elements. For each element, this number must be a number that is averaged over all of its isotopes according to their relative percent natural abundance. The atomic weight of an element, then, is the average atomic mass of all of the element’s naturally occurring isotopes. The molecular mass then becomes the sum of the atomic weights comprising the molecule, according to the number of each kind of atom occurring in the molecule. In other words, the molecular mass is the sum of the weights of the atoms represented in a molecular formula. Molecular masses (also called molecular weights) are the masses of molecules, which consist of essentially covalent compounds, while formula masses (also called formula weights) are the masses of formula units, which are essentially ionic compounds. The unit, in either case, is the amu, but often converted to the more useful grams/mole, which has the same numerical value.
Basic Atomic and Nuclear Physics
Published in Douglas S. McGregor, J. Kenneth Shultis, Radiation Detection, 2020
Douglas S. McGregor, J. Kenneth Shultis
Most naturally occurring elements are composed of two or more isotopes. The isotopic abundance γi of the i-th isotope in a given element is the fraction of the atoms in the element that are that isotope. As with the elemental relative atomic mass, the isotopic abundances of a sample depends on the origins of the sample. For a specified element, the elemental atomic weight is the weighted average of the atomic weights of all naturally occurring isotopes of the element, weighted by the isotopic abundance of each isotope, i.e., A=∑iγi(%)100Ai, where the summation is over all the isotopic species comprising the element.
Nuclear Terrorism
Published in Robert A. Burke, Counter-Terrorism for Emergency Responders, 2017
The smallest part that any element can be reduced to is an atom of that element. An atom can also be broken down into its component parts, sometimes referred to as subatomic particles. Atoms contain a nucleus and varying orbitals of electrons around the outside of the nucleus. Generally, chemical reactions involve the electrons in the outer shell of any given element. Radioactivity, on the other hand, involves the nucleus of the atom. Within the nucleus, there are two subatomic particles, protons and neutrons. Protons have a positive charge, while neutrons do not have a charge and are considered neutral. Electrons orbiting around the outside of the nucleus are negatively charged. An atom must be electrically balanced and therefore must have an equal number of negative charges in the electrons orbiting the nucleus as there are positive-charged protons in the nucleus. Numbers of protons within the nucleus of atoms vary from element to element. It is, in fact, the number of protons within an element's nucleus that identifies that element. The atomic number of an element is the number of protons present in the nucleus of an atom. The number is located on the periodic table of elements and is the whole number by the element without any decimal points. Atoms of the same element may contain varying numbers of neutrons in the nucleus. When the number of neutrons differs from the normal, but the number of protons is the same, it is said to be an isotope of that element. Atomic weight or mass of an element is determined by the combined weights of the protons and neutrons in the nucleus. Neutrons and protons of an element are represented on the periodic table by the atomic weight. This is the number by the element symbol with a decimal point.
Values in the Development of Early Periodic Tables
Published in Ambix, 2020
Mendeleev concluded this evaluation of potential organising principles with the foreseeable point that only atomic weights offered a firm foundation for systematisation. As atomic weight was a fundamental property of the elements, studying the relationship between atomic weights and other properties of the elements was of importance. But as he was not the first one to engage in such a study, he saw it appropriate to review the existing attempts towards this direction. Perhaps as a response to Meyer, Mendeleev started by considering the relationship between atomic weights and valency. He argued that the studies of valency expressed uncertain results. From the point of view of valuing completeness, Mendeleev’s diagnosis of the uncertainty of the studies on valency is telling; he worried that it had been investigated in isolation from other properties of elements. He recommended to study the relationship between many properties of elements and their atomic weights, not just one. This emphasis on focussing on “many” over the “few” demonstrates how Mendeleev sought to foreshadow the greater completeness of his system.71
Estimation of Absorbed Dose Due to Gas Bremsstrahlung Based on Residual Gas in Electron Storage Rings
Published in Nuclear Science and Engineering, 2023
Akihiro Takeuchi, Masayuki Hagiwara, Hiroki Matsuda, Toshiro Itoga, Hiroyuki Konishi
where the subscript i is the residual gas composition element, the subscript k is the residual gas composition molecule, is the partial pressure ratio, and is the number of moles. All elements in the compound are decomposed into monoatoms, and the average atomic weight expressed as a single element is obtained from the atomic number ratio for each element.