Accurate relative atomic masses

With the development of high resolution mass spectroscopy the mass of the molecular fragment can be measured to seven significant figures. These very accurate relative atomic masses make it possible to distinguish molecules with very similar molecular mass values. 

For example, working in molar terms, the accurate relative atomic mass of the fluorine atom is 18.9984032 and the mass number is 19. The mass numbers of protons and neutrons are 1 and the 9 protons and 10 neutrons of the fluorine nucleus give a total of 19, the electron masses being too small to make any difference. Using accurate figures for the terms in equation (1.1), ie. those from Table 1.1 ... 

Accurate atomic weight values do not automatically follow from precise measurements of relative atomic masses, however, since the relative abundance of the various isotopes must also be determined. That this can be a limiting factor is readily seen from Table 1.3 the value for praseodymium (which has only 1 stable naturally occurring isotope) has two more significant figures than the value for the neighbouring element cerium which has 4 such isotopes. In the twelve years since the first edition of this book was published the atomic weight values of no fewer than 55 elements have been improved, sometimes spectacularly, e.g. Ni from 58.69( 1) to 58.6934(2). 

Atomic weights are known most accurately for elements which have only 1 stable isotope the relative atomic mass of this isotope can be determined to at least 1 ppm and there is no possibility of variability in nature. There are 20 such elements Be, F, Na, Al, P, Sc, Mn, Co, As, Y, Nb, Rh, I, Cs, Pr, Tb, Ho, Tm, Au and Bi. (Note that all of these elements except beryllium have odd atomic numbers — why )... 

Table B.l summarizes the ground-state electron configuration and formal APH indices (turn number t, angular number l-n) for each known element, together with atomic number (Z) and relative atomic mass). As shown by the asterisks in the Anal column, 20 elements exhibit anomalous electron configurations (including two that are doubly anomalous - Pd and Th), compared with idealized t/l-n APH descriptors. These are particularly concentrated in the first d-block series, as well as among the early actinides. Such anomalies are indicative of configurational near-degeneracies that may require sophisticated multi-reference approximation methods for accurate description.

A little recognized systematic error in the calculation of accurate masses of, for example, small radical cation molecular ions (as in electron ionization (El)) or protonated molecular ions (as seen in the soft ionization methods) is the fact that the electron has a small, but finite mass. The accurate masses of radical cations, in which a valence electron has been removed, of anions that have been created by capture of an electron, and of protonated species produced by soft ionization processes, should take into consideration this small mass difference [19]. For example, there is a small difference between the relative atomic mass of a neutral hydrogen atom and a proton. The accepted accurate mass of 1H° is 1.007825 Da. The accurate mass of 1H+ is 1.007276 Da. To be completely correct, expected accurate masses of protonated molecular ions, [M+H]+, produced by electrospray should be calculated using the mass of one H+, rather than all of neutral hydrogen atoms. Mamer and Lesimple do acknowledge, however, that, for large molecules, the error is of little consequence. 

The masses of isotopes can be measured with accuracies better than parts per billion (ppb), e.g., m40Ar = 39.9623831235 0.000000005 u. Unfortunately, determinations of abundance ratios are less accurate, causing errors of several parts per million (ppm) in relative atomic mass. The real limiting factor, however, comes from the variation of isotopic abundances from natural samples, e.g., in case of lead which is the final product of radioactive decay of uranium, the atomic weight varies by 500 ppm depending on the Pb/U ratios in the lead ore. [8]... 

Table A. 1 comprises the stable elements from hydrogen to bismuth with the radioactive elements technetium and promethium omitted. Natural variations in isotopic composition of some elements such as carbon or lead do not allow for more accurate values, a fact also reflected in the accuracy of their relative atomic mass. However, exact masses of the isotopes are not affected by varying abundances. The isotopic masses listed may differ up to some 10 u in other publications.

Mass spectrometers measure the relative mass of an individual ion. with its particular combination of isotopes. We therefore use the relative atomic masses of the most abundant isotopes lo calculate the accurate relative molecular mass of an ion. 

Fundamental atomic and physical properties of the alkali metals are given in Tables 1, 2, and 3. The elements are characterized by having electron configurations each with a single s orbital electron outside a noble gas core (see Table 1). Sodium and cesium are mononucUdic so that their relative atomic masses are known extremely accurately in effect, the same can be written for potassium and rubidium since their isotopes (of which there are three and two, respectively) have... 

Gravimetric analysis is the process of converting an element into a definitive compound, isolating this compound from other constituents in a sample and then weighing the compound (Box 20.1). The weight of the element can then be calculated from the formula of the compound and the relative atomic masses of the elements involved. You need to be able to weigh accurately, by difference, a substance to four decimal places (see p. 23). 

As in the lUPAC report, values in parentheses are used for radioactive elements whose relative masses cannot be quoted exactly without knowledge of the origin of the elements the value given is the atomic mass number of the isotope of that element of longest known half life. Alternatively, the relative atomic mass of the isotope with the longest known half life or that of a commonly used isotope could be listed, since the relative atomic masses of the individual isotopes are accurately known. 

Berzelius, Jons Jacob (1779-1848) Swedish chemist who had a major influence on the development of modern chemistry. He introduced the symbols now used to denote chemical elements and coined a number of words still used in chemistry, such as catalysis , halogen , isomerism , and protein . He also determined relative atomic masses (atomic weights) accurately, thereby helping establish Dalton s atomic theory. Berzelius was also involved in the discovery of several elements cerium, siU-con, selenium, and thorium, and the introduction of some experimental apparatus, such as rubber tubes and filter paper, into chemistry. 

A mass spectrometer (Figure 2.23) allows chemists to determine accurately the relative atomic masses of atoms. It can also be used to determine the relative molecular masses of molecular compounds and establish their structure. 

In the context of general chemistry we rarely pay attention to the different isotopes of the individual elements involved in a reaction. For instance, the molecular mass of tribromomethane, CHBrs, is usually calculated as 252.73 g moF on the basis of relative atomic mass from the Periodic Table. In mass spectrometry, however, we need to more accurately consider individual isotopes, b ause mass spectrometry is based upon the separation of ions by mass-to-charge ratio, m/z- [1-3]. Thus, there actually is no signal at m/z 252.73 in the mass spectrum of tribromomethane. Instead, major peaks occur at m/z 250, 252, 254, and 256 accompanied by some minor other ones. 

We can use the data obtained from a mass spectrometer to calculate the relative atomic mass of an element very accurately. To calculate the relative atomic mass we follow this method ... 

High-resolution mass spectrometers can distinguish between ions that appear to have the same mass on a low-resolution mass spectrum. Table 29.5 shows the accurate relative isotopic masses of the most common atoms found in organic molecules. 

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