Relative atomic mass table

Each element that has neither a stable isotope nor a characteristic natural isotopic composition is represented in this table by one of that element s commonly known radioisotopes identified by mass number and relative atomic mass. 

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). 

This table is scaled to the relative atomic mass Ar(13C) = 12. 

One problem with Mendeleev s table was that some elements seemed to be out of place. For example, when argon was isolated, it did not seem to have the correct mass for its location. Its relative atomic mass of 40 is the same as that of calcium, but argon is an inert gas and calcium a reactive metal. Such anomalies led scientists to question the use of relative atomic mass as the basis for organizing the elements. When Henry Moseley examined x-ray spectra of the elements in the early twentieth century, he realized that he could infer the atomic number itself. It was soon discovered that elements fall into the uniformly repeating pattern of the periodic table if they are organized according to atomic number, rather than atomic mass. 

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.

Table B.l. The currently known chemical elements, showing atomic number (Z), chemical symbol, name, relative atomic mass, ground-state electron configuration, and APH indices (t = turn number l-n = angular number) asterisks (, ) symbolize anomalous (APH non-conforming) ground-state electronic configurations, which are indicative of configurational near-degeneracy...

Mass spectrometry is based upon the separation of charged ionic species by their mass-to-charge ratio, m/z. Within the general chemical context however, we are not used to taking into concern the isotopes of the elemental species involved in a reaction. The molecular mass of tribromomethane, CHBrs, would therefore be calculated to 252.73 g mol using the relative atomic masses of the elements as listed in most periodic tables. In mass spectrometry we have to leave this custom behind. Because the mass spectrometer does not separate by elements but by isotopic mass, there is no signal at m/z 252.73 in the mass spectmm of tribromomethane. Instead, major peaks are present at m/z 250, 252, 254 and 256 accompanied by some minor others. 

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.

Table A.l. Isotopic mass, isotopic composition, and relative atomic mass [u] of non-radioactive elements. lUPAC 2001.

MENDELEEV USED KNOWN RELATIVE ATOMIC MASSES TO CREATE THE PERIODIC TABLE... 

One problem with Mendeleev s table was that some elements seemed to be out of place. For example, when argon was isolated, it did not seem to have the correct mass for its location. Its relative atomic mass of 40 is the same as calcium s, but argon is an inert gas and calcium a reactive metal. Such anom-... 

Next find the relative atomic mass of sodium from the Periodic Table (p. 136) and work out the relative formula masses of water, sodium hydroxide and hydrogen gas. 

A simple periodic table contains the symbols, atomic numbers and the relative atomic masses of the elements. Additionally, detailed periodic tables containing some physical and chemical properties (such as melting point, boiling point, oxidation state) are also made. 

Some of the adjusted constants used in the 1998 least-squares adjustment are given in Table 1. In that table, the relative atomic masses are defined by... 

For molecules Mr is the relative molecular mass or molecular weight for atoms Mr is the relative atomic mass or atomic weight and the symbol Ar may be used. Mr may also be called the relative molar mass, Mr>B = Mb/M, where M = 1 gmol-. The standard atomic weights, recommended by IUPAC, are listed in table 6.2, p.94. 

Table 6.2 lists the relative atomic masses of the elements in the alphabetical order of chemical symbols. The values have been recommended by the IUPAC Commission on Atomic Weights and Isotopic Abundances in 1991 [44] and apply to elements as they exist naturally on earth. 

The relative atomic masses of many elements depend on the origin and treatment of the materials [45]. The notes to this table explain the types of variation to be expected for individual elements. When used with due regard to the notes the values are considered reliable to the figure given in parentheses being applicable to the last digit. For elements without a characteristic terrestrial isotopic composition no standard atomic weight is recommended. The atomic mass of its most stable isotope can be found in table 6.3. 

IX.8 PERIODIC TABLE OF THE ELEMENTS The relative atomic masses have been rounded off to a maximum of 4 significant... 

The relative formula mass (RFM) equals the atomic or molecular weight of the substance, so for a compoimd such as sodium chloride (NaCl) this is simply the sum of the atomic weights of Na (23) and Cl (35.5), taken from the Periodic Table of the elements (see Appendix 2). The terms relative formula mass and relative atomic mass are often taken as interchangeable. 

In other words, the relative atomic mass of an element is the average mass of its atom to 1/12 of the mass of a atom. In the periodic table, the masses of atoms are written according to these relative calculations. For compounds similar to relative atomic mass, a relative formula mass is used. A relative formula mass is the sum of relative atomic masses of the atoms found in a compound. 

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... 

Chromium has the atomic number 24. Of the 13 known isotopes (mass numbers 45-57), four are stable, giving chrominm the relative atomic mass 51.9961 ( C = 12.0000). Table 1 lists the properties of some isotopes. Cr may be used for NMR spectroscopy however, its relative receptivity (8.62 x 10 , H= 1.00), quadmpole moment (4.1 x 10 ° m ), and low resonance frequency (16.956MHz, H = 300MHz) pose experimental difficulties. Radioactive Cr is used in medical tracer studies. 

Nickel (relative atomic mass = 58.69) is a first-row transition metal in group Vlllb of the periodic table. Although there are five natural isotopes, Ni (68.3%) and Ni (26.1%) are the most abundant. Proteins are often labeled with Ni (nuclear spin, I, is 3/2) to identify the nickel site by EPR spectroscopy, since paramagnetic species with significant spin on Ni exhibit a four-line splitting. Ni, which is a beta emitter with a relatively long half-life of 100 years, can be used to identify Ni-containing proteins and to measure Ni uptake in cells. 

The metallic element thallium, atomic number 81 (relative atomic mass 204.38), is the heaviest member of group 13 of the periodic table. The ground-state electron configuration is [XeJdf Sd Obs bp. ... 

From the Table of Weights 1989 by ihe Commissioo on Atomic Weights, lUPAC (Ft/ef, Appl. Chem. 1991.63.97S-10023. Weights are scaled to the relative atomic mass A/ Q 12. 

Using the periodic table, what is the relative atomic mass of... 

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