Atomic numbers/weights tabulated

When Mendeleev (pronounced "men-del-lay-ev") and Meyer developed their tabulations, the concept of atomic number was not yet known. Their classifications were based on the atomic weights known at that time. 

A modem version of the periodic table, with the elements arranged by atomic number, is shown in Figure 2.15 (see also inside front covct). Each entry lists the atomic number, atomic symbol, and atomic weight of an element Ibis is a convaiiait way of tabulating such information, and you should become familiar with using the pmodic table for that purpose. As we develop the subject matt of chemistry throughout the text, you will see how useful the periodic table is. 

Element has no stable nuclides the value given in parentheses is the atomic mass number of the isotope of longest known half-life. However, three such elements (Th, Pa and U) do have a characteristic terrestrial isotopic composition, and for these an atomic weight is tabulated. 

The tabulated molar mass of an element divided by Avogadro s number is the average mass per atom of that element, but it is not the exact mass of an individual nucleus. There are two reasons for this. First, molar masses refer to neutral atoms. The tabulated molar mass of an element includes the mass of its electrons in addition to the mass of its nucleus. Consequently, the mass of Z electrons must be subtracted from the isotopic molar mass in computing the energy of formation of a nuclide. Second, molar masses of the elements are weighted averages of... 

When a combustible substance is mixed with air, the mixture will explode only when it is neither too rich nor too lean. The lower explosion limit (LEL) is the minimum volume percent of the substance in air with flammability, which is separated from the upper explosion limit (UEL) by the explosive concentration range. The tabulations in handbooks are based on experimental data, and sometimes derived from estimation methods based on the elemental composition of the fuel as CmEtxOy. Figure 6.11 shows the LEL for the series of normal paraffins and of 1-alcohols versus the number of carbon atoms. There are two ways to plot the results, which show that, for paraffins, the volume percent shows a steeply declining trend, but the weight percent shows a mildly increasing trend. One may conclude that a smaller volume percent of higher paraffin... 

Inspection of the periodic table reveals that the mass number of chlorine is actually 35.45 amu, not 35.00 amu. The existence of isotopes accounts for this difference. A natural sample of chlorine is composed principally of two isotopes, chloiine-35 and chlorine-37, in approximately a 3 1 ratio, and the tabulated mass is the weighted average of the two isotopes. In our calculation the chlorine atom referred to was the isotope that has a mass number of 35 amu. 

Many of the isotope masses have been determined by MS accurately to seven or more decimal places. Numbers in parentheses in the atomic weights represent the first uncertain figure. Atomic weights based on the values in Pure Appl.Chem. 2001, 73, 667-683 as tabulated at http //www.chem.qmul.ac.uk/iupac/AtWt/. Used with permission. 

Atomic weight (1820) (atomic mass) n. The mass of an elemental isotope relative to that of the C-12 isotope of carbon, whose mass has been set at exactly 12.0000 atomic units (daltons). For most elements, the tabulated atomic weight is the average, weighted by natural mass abundance, over all the element s isotopes, so is never an integer. The actual mass-fraction of a gram-of one a u is the reciprocal of Avoga-dro s number. 1/(6.02283 x 10 ). 

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