Atomic weight or mass

What was the scientist s new picture of the structure of the atom in 1932 The idea of planetary electrons outside the nucleus remained unchanged. But his conception of the nucleus was different. There were no longer any free electrons in the nucleus. Only protons and neutrons were found there. The atomic weight or mass of an element was equal to the total number of protons and neutrons in its atom. The atomic number of an element was defined either as the number of planetary electrons in its atom, or as the number of protons in the nucleus of its atom. 

The atomic weight (or mass) of an element is given by the weighted average of the isotopes masses. Isotopes are atoms of an element that have different masses. 

Instruments known as spectroscopes or spectrometers are generally used for elemental analyses. Spectroscopic methods examine the interaction between atoms and light and are defined primarily by the portion of the electromagnetic spectrum that they use. Instead of the spectrum, more powerful mass spectrometers use electromagnetic fields to sort atoms according to their weight. Specific atomic weights, or masses, correspond to specific elements and isotopes. 

Let us talk about some concepts of nuclear physics. We have discussed isotopes. Now we meet another term—isobars —elements having the same atomic weights or mass numbers but different atomic numbers (from the Greek for heavy ). Isobars, in other words, are isotopes of different... 

The weight of atoms and their constituents can be given in kilograms. A proton, for example, weighs 1.67 x 10 kilograms, but its weight or mass can be expressed more conveniently in a measure called the atomic mass unit (amu). One amu is defined as 1/12 the mass of a carbon atom that consists of six protons. 

Lu Q, Masuda A (1994) The isotopic composition and atomic-weight or molybdenum, hit J Mass Spectr Ion Proc 130 65-72... 

As an atom is extremely small, it is not possible to establish its weight (or mass) directly. The atomic mass or weight of an element is established indirectly. Dalton selected hydrogen... 

Molecular weight (or mass) The sum of all the atomic weights (masses) of the atoms in the molecule. 

All of the weight relations in chemical reactions depend upon the weights of the atoms of the elements. These weights (or masses), called atomic weights, are ery important in the stud> and practice of chemistry. 

The atomic weight or relative atomic mass of an element is the mass of one atom of that element relative to that of the most abundant form of carbon taken as 12 units. On this scale the atomic weight of hydrogen is 1, oxygen is 16, and copper is 63.54a.m.u. Table 1.1 lists the atomic weights of the first 18 elements of the Periodic Table. 

Each nucleus is characterized by a definite atomic number Z and mass number A for clarity, we use the symbol M to denote the atomic mass in kinematic equations. The atomic number Z is the number of protons, and hence the number of electrons, in the neutral atom it reflects the atomic properties of the atom. The mass number gives the number of nucleons (protons and neutrons) isotopes are nuclei (often called nuclides) with the same Z and different A. The current practice is to represent each nucleus by the chemical name with the mass number as a superscript, e.g., 12C. The chemical atomic weight (or atomic mass) of elements as listed in the periodic table gives the average mass, i.e., the average of the stable isotopes weighted by their abundance. Carbon, for example, has an atomic weight of 12.011, which reflects the 1.1% abundance of 13C. 

There is a difference between the mass of an atom (or ion) and what chemists are used to thinking of as the atomic weight or MW of a species. Atomic weight is the average weight of all... 

The mass spectrum gives us all of the information needed to calculate the atomic weight the masses of isotopes and their relative numbers, or fractional abundances. The fractional abundance of an isotope is the fraction of the total number of atoms that is composed of a particular isotope. The fractional abundances of the neon isotopes in naturally occurring neon are neon-20, 0.9051 neon-21, 0.0027 and neon-22, 0.0922. 

Ay, = Atomic weight or molar mass g/mol) t = Time (s) z = Valence... 

The atomic weight of an element or the molecular weight of a compound may be specified on the basis of amu per atom (molecule) or mass per mole of material. In one mole of a substance, there are 6.022 X 10 (Avogadro s number) atoms or molecules. These two atomic weight schemes are related through the following equation ... 

Clearly, there is one molecular property that can be exactly calculated fi om the contributions of its constituent atoms the molecular weight, or, more correctly, the molecular mass, which is exactly the sum of the masses of its constituent atoms. 

Each of the elements has a number of isotopes (2,4), all radioactive and some of which can be obtained in isotopicaHy pure form. More than 200 in number and mosdy synthetic in origin, they are produced by neutron or charged-particle induced transmutations (2,4). The known radioactive isotopes are distributed among the 15 elements approximately as follows actinium and thorium, 25 each protactinium, 20 uranium, neptunium, plutonium, americium, curium, californium, einsteinium, and fermium, 15 each herkelium, mendelevium, nobehum, and lawrencium, 10 each. There is frequently a need for values to be assigned for the atomic weights of the actinide elements. Any precise experimental work would require a value for the isotope or isotopic mixture being used, but where there is a purely formal demand for atomic weights, mass numbers that are chosen on the basis of half-life and availabiUty have customarily been used. A Hst of these is provided in Table 1. 

Elements with 1 predominant isotope can also, potentially, permit very precise atomic weight determinations since variations in isotopic composition or errors in its determination have a correspondingly small effect on the mass-spectrometrically determined value of the atomic weight. Nine elements have 1 isotope that is more than 99% abundant (H, He, N, O, Ar, V, La, Ta... 

One gram-atom (or pound-atom) is the mass in grams (or pounds) of a given element that is numerically equal to its atomic weight. Thus, the number of gram-atoms of an elementary substance is m/A, where m is the mass (in grams) and A, its atomic weight. 

Equation 1-5 was written for a sample containing a single element upon which monochromatic x-rays are incident. In so far as x-ray absorption is an atomic property, the mass absorption coefficients for other samples are additive functions of the weight-fractions of the elements, free or combined, that are present that is,... 

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