Chemical atomic weights

Let us do a little bookkeeping with the exact masses of these nuclei. Actually we will simplify a bit and use the exact masses of the atoms. This will make no difference. The masses of the atoms differ from the nuclear masses by the masses of the number of electrons in each atom. We have shown that electrons are conserved in nuclear changes. Exact masses of atoms (that is, exact masses of each isotopic species and not the chemical atomic weights shown on the inside back cover) are readily available. For our hydrogen-helium reaction we have... 

Boltzmann s constant k Gas constant R Bohr magneton Ratio of physical to chemical atomic weights Energy of 1 ev Energy of 1 ev... 

The following values of isotopic atomic weights and abundances are obtained with a mass spectrometer. Compute the chemical atomic weights for the elements involved. The isotopic weights are in parentheses. 

The following table gives the chemical atomic weights of several important elernents, found in this way. It is customary to take the chemical atomic weight of oxygen to be sixteen, so that for example, that of sulphur is 32, since an atom of sulphur is twice as heavy as one of oxygen, and the atomic... 

CHEMICAL ATOMIC WEIGHTS TAKING THAT OF OXYGEN TO BE 16... 

Isotope Chemical atomic weight of elements in nature... 

The chemical atomic weights of elements are the average relative weights (masses) of atoms of the elements, the average being for the usual isotopic composition of eUch element ... 

Prout s hypothesis was revived by the discovery of isotopes thus chlorine con sists of two natural isotopes and CP" , and boron of two isotopes B and in each case with nearly integral atomic weights and present in such relative amounts as to give the chemical atomic weight. It is now seen that Prout s idea e ntained an element of truth. 

For an element consisting of several isotopes the chemical atomic weight is found by determining the masses of the different isotopes and also their relative amounts. The way that this is done is indicated in Figure 7-4, which shows the results of a mass-spectrographic investigation of nickel. The isotopic composition of this element is found in this way to be Ni , 67.4% 26.7% ... 

Equation 5 can be simplified by replacing the number of atoms N by W /M, where W is the weight of the element present, is the abundance of the isotope activated, and M the chemical atomic weight. If o-, the activation cross section, is expressed in barns (1 bam is equal to 10 square centimeters) it reduces to... 

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. 

Since naturally occurring elements consist of mixtures of isotopes, the chemical atomic weights will be an average of the isotope weights of each element, taking into account their relative naturally occurring abundances. Thus, none of the... 

To calculate the chemical atomic weight of hydrogen from data obtained by the mass-spectrograph. 

We conclude that the chemical atomic "weight of hydrogen lies between 1.00799 and 1.00804. 

In 1961, however, international organizations of both chemists and of physicists agreed to adopt an atomic weight standard based on carbon-12 set equal to exactly 12.0000. This new standard was almost exactly that of the old chemical atomic weights and yet it was tied to a single isotope and not to the average of a group of them. 

Atomic- Number Z Name Chemical Atomic Weight A Denaity (P) gjyfcc, 20 C 760 an Hg Nuclei per cc (X 10-34) N (l-b) Arg. Lag Energy Ipsa ( n Neut ermal ron Cr cr) in (0.025 ev) MS Sections )sms Ibermal (0.025 ey) Macroscopic Cross Sections, oN Ibermal (0.025 er) Mean Free Path cm Slowing Down Power No-gf Moderating Ratio (crs/cr ) Xf... 

Dumas determined the vapour densities of mercury, phosphorus, arsenic hydride, and stannic and titanium chlorides, the last two being redetermined in 1830, and the vapour density of phosphorus again (with those of iodine and sulphur), and again in 1832. The vapour density of phosphorus was 4-355 or 4 420 (air = i) and that of sulphur at 493° 6 495, at 506 " 6 512, at 524° 6 617 and 6 581. The vapour density of phosphorus, he found, corresponded with four times, that of sulphur with six times, and that of mercury with half, the chemical atomic weights. Since Dumas thought that the molecules of all elementary gases contain two atoms, he was unable to resolve these difficulties. 

Berzelius who first used symbols to name elements, coined the word isomer through his work with chemical atomic weights. Not to be confused with isotopes, chemical element isomers have the same molecular formula, but different structural configurations. 

John Dalton chose the value 1 for hydrogen as the base of his scale of atomic masses. The Swedish chemist J. J. Berzelius used 100 for oxygen, and the Belgian chemist J. S. Stas (1813-1891), who carried out many quantitative analyses of compounds, proposed 16 for oxygen (the natural mixture of isotopes), and this base was used for many years. For several decades nucleidic masses were expressed on a scale (called the physical scale) based on iVth the mass of the neutral atom " O the chemical atomic-weight unit was then 1.000272 times the physical atomic-mass unit. This period of confusion was brought to an end in 1961 by the acceptance of iVth the mass of -C as the unit for both atomic masses and nucleidic masses. 

A chemically pure organic compound will give a mixture of mass spectra because the elements that compose it are not isotopically pure. Recall the case of the element neon, which Sir J. J. Thomson (1913) showed gave not one peak at its chemical atomic weight of 20.2, but two peaks at masses 20.0 and 22.0, in relative abundances of 10 1. Of the common elements encountered in organic compounds (shown in Table 2.1, inside the front cover), many have more than one isotope of appreciable natural abundance. The isotopic abundances of other... 

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