Compounds relative weights

Toxic equivalency factors (TEFs) are estimated relative to 2,3,7,8-TCDD, which is assigned a value of 1. They are measures of the toxicity of individual compounds relative to that of 2,3,7,8-TCDD. A variety of toxic indices, measured in vivo or in vitro, have been used to estimate TEFs, including reproductive effects (e.g., embryo toxicity in birds), immunotoxicity, and effects on organ weights. The degree of induction of P450 lAl is another measure from which estimations of TEF values have been made. The usual approach is to compare a dose-response curve for a test compound with that of the reference compound, 2,3,7,8-TCDD, and thereby establish the concentrations (or doses) that are required to elicit a standard response. The ratio of concentration of 2,3,7,8-TCDD to concentration of test chemical when both compounds produce the same degree of response is the TEF. Once determined, a TEF can be used to convert a concentration of a dioxin-like chemical found in an environmental sample to a toxic equivalent (TEQ). 

A great deal of difficulty was encountered at first, because Dalton s fifth postulate gave an incorrect ratio of numbers of atoms in many cases. Such a large number of incorrect results were obtained that it soon became apparent that the fifth postulate was not correct. It was not until some 50 years later than an experimental method was devised to determine the atomic ratios in compounds, at which time the scale of relative atomic weights was determined in almost the present form. These relative weights are called the atomic weights. 

Figure 8.22 highlights the fraction of naphthalenes present in water due to colloidal entrainment. As the compound molecular weight increases, the relative... 

The English chemist John Dalton became one of the most famous scientists of the eighteenth century. Although he was known to the public for one idea, that chemical compounds were formed when the atoms of one element joined with the atoms of another, there was much more than this to Dalton s theory. He revolutionized chemistry by emphasizing that atoms have relative weights and that these relative weights can be measured. 

Perhaps it is worth emphasizing again that the importance of Dalton s theory didn t lie in the assumption that matter is composed of indestructible atoms. That was a very old idea. On the contrary, his idea was significant because it was a theory that explained how chemical compounds are formed and because the idea of atoms with different relative weights made it possible to turn chemistry into a quantitative science. As long as chemists held on to the old idea that elements could combine with one another in a variety of different proportions, they could describe chemical reactions only in a qualitative manner. It was Dalton who changed all this. 

Berzelius s own experiments soon convinced him that Dalton was right to conclude that atoms always combine with one another in small whole-number ratios. Berzelius realized that determining the relative weights of all of the elements would be of enormous value to chemistry, because it would then be possible to determine the exact composition of any chemical compound. Without work of this kind, he said, no day could follow the morning dawn. Because he knew of no other chemist who was pursuing this line of research, he decided to do it himself. 

The supposition that changes in the properties of substances are connected with changes in the numbers, movements, and arrangements of different kinds of minute particles, was used in a general way by many naturalists of the 17th and 18th centuries but Dalton was the first to show that the data obtained by the analyses of compounds make it possible to determine the relative weights of the atoms of the elements. 

As a result of his work on relative weights, Dalton formulated the Law of Multiple Proportions, which states that when elements combine to form more than one compound, then the ratio of the masses of elements in the compounds are small whole number ratios of each other. For example, the elements carbon and oxygen form the two compounds carbon monoxide (CO) and carbon dioxide (CO ). The ratio of... 

Dalton s work on relative weights, multiple proportions, and the atomic theory did not have an immediate effect on chemists of his day. Dalton s ideas did provide a framework for determining the empirical formula of compounds, but his table of relative weights was not accurate enough to give consistent results. Many scientists still debated the existence of atoms in the second half of the nineteenth century. Still, little by little, the atomic theory was adopted by chemists as a valid model for the basic structure of matter. While Dalton continued his life as a humble tutor in Manchester, other chemists used Dalton s ideas to establish the atomic theory. Foremost among these was Jons Jacob Berzelius (1779-1848) of Sweden, the foremost chemical authority of the first half of the nineteenth century. 

Dalton went on to explain how these weights can be obtained. It is clear that if one knew the number of atoms in the molecule of a compound, the relative weights of the different atoms could be calculated from the empirically measured composition. Daltons procedure was to provide a set of rules of simplicity by which the formula could be reasonably assumed. 

The first part of Daltons book provided very few examples of compositions expressed in atomic formulas. Nor did he elaborate on his claim, quoted above, that from the relative weights of the ultimate particles or atoms of the bodies. .. their number and weight in various other compounds would appear, in order to assist and to guide future investigations, and to correct their results.The evidence confirming the real usefulness of Daltons conception of atomic weights was first provided by other more experienced chemists. 

Here, L2 represents the chemical property space corresponding to the VCL, d(x, rf) represents an appropriate distance metric between the lead compound ry and the compound , p xf is the relative weight that can be attached to compound (if all compounds are of equal importance, then the weights p(xy) = for each i), and Mis typically much smaller than N. That is, this problem seeks a subset of M lead compounds ry in a descriptor space such that the average distance of a compound from its nearest lead compound is minimized. Alternatively, this problem can also be formulated as finding an optimal partition of the descriptor space co into M clusters Ay and assigning to each cluster a lead compound rj such that the following cost function is minimized ... 

Fig. 4.3. (a) Locations x, 1 < i < 5,000, of compounds (circles) and rj, 1 < j < 12, of lead compounds (crosses) in the 2-d descriptor space determined from the original algorithm, (b) Relative weights Xj associated with different locations of lead compounds. Reprinted ( adapted or in part ) with permission from Journal of Chemical Information and Modeling. Copyright 2008 American Chemical Society. 

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