Compounds symbolic representation

From this description, however, alternative (and/or complementary) presentations of the structure and different symbolic representations can be deduced. These are often differently defined for specific groups of compounds and may be useful to obtain a clearer view of the atomic assembly and/or to make an easier comparison between different compounds. In other words it must be underlined that there is no ideal way of describing all structure types. The most appropriate way of description depends on the structure itself but also on a number of points we are interested in emphasizing (comparison with other structural types, comparison with other compounds of the same element, etc.). These points will be discussed in a few subsequent sections after the presentation of the conventional description. 

Lewis formula (for an ionic compound) A representation of the structure of an ionic compound showing the formula unit of ions in terms of their Lewis symbols. Lewis structure A diagram showing how electron pairs are shared between atoms in a molecule. Examples H-C1 0=C=0. 

Fullerenes can encapsulate various atoms within the cages, and these compounds have been referred to as endohedral fullerenes. For example, the symbolic representations La C6o and La2 Cso indicate that the fullerene cage encapsulates one and two lanthanum atom(s), respectively. The IUPAC description refers to these fullerenes species as incar-fullerenes, and the formulas are written as t LaCeo and tl Cso, (i is derived from incarcerane). Some metal endohedral fullerenes are listed in Table 14.2.1. The endohedral fullerenes are expected to have interesting and potentially very useful bulk properties as well as a fascinating chemistry. Some non-metallic elements, such as N, P, and noble gases, can be incarcerated into fullerenes to form N 0,0, P C6o, N C o, Sc3N C80, Ar Oo, etc. 

Besides his table of atomic masses, Berzelius made many other major contributions to chemistry. The most important of these was the invention of a simple set of symbols for the elements along with a system for writing the formulas of compounds to replace the awkward symbolic representations of the alchemists. Although some chemists, including Dalton, objected to the new system, it was gradually adopted and forms the basis of the system we use today. 

Compounds arise from the combination of atoms in fixed ratios. In any such combination, the resultant substance behaves differently from the atoms alone. In many compounds, atoms combine to form discrete particles called molecules. Molecules can be broken down into their constituent atoms, but the resulting collection of atoms no longer behaves like the original molecule. Other materials are composed of vast arrays or extended structures of atoms or ions but do not form discrete molecules. Alloys, metals, and ionic solids (composed of paired ions) fall into this category of chemical compounds. We ve seen how we can use atomic symbols as shorthand notation to designate atoms. That same idea can be extended to describe the composition of either molecules or extended compounds in a simple symbolic representation. 

These statements, which contain the whole of Dalton s chemical atomic theory, were arrived at completely in September to October 1803, and remained unchanged in all Dalton s later publications. The symbolic representation of the compositions of compounds used by Dalton (his symbol law ) contained implicitly the laws of definite, multiple, and reciprocal proportions, and Dalton does not give verbal statements of these laws. It was asserted that the law of constant proportions would be a better name than the law of definite proportions and that a further law of compound proportions (vi) is necessary, stating that the combining weight of a compound is the sum of the combining weights of its components, which does not follow from the other laws of chemical combination. 

The product, in addition to helium-4, is thorium-234. This is an example of a nuclear equation, which is a symbolic representation of a nuclear reaction. Normally, only the nuclei are represented. It is not necessary to indicate the chemical compound or the electron charges for any ions involved, because the chemical environment has no effect on nuclear processes. 

Compounds are pure substances that are special combinations of two or more elements. The elements that are present in this combination are shown in a symbolic representation for the compound, which is known as its chemical formula, or simply its formula. The compound that is probably most familiar to everyone is water. Water is a special combination of the elements hydrogen and oxygen, which is shown in the formula for water, HjO. Other examples of compounds and their formulas are sodium chloride (formula, NaCl), a special combination of the elements sodium (Na) and chlorine (Cl) calcium carbonate (formula, CaCOj), a special combination of the elements calcium (Ca), carbon (C), and oxygen, (O) and potassium chromate (formula, K2Cr04), a special combination of the elements potassium (K), chromium (Cr), and oxygen (O). 

A molecular compound is made up of discrete units called molecules, which typically consist of a small number of nonmetal atoms held together by covalent bonds. Molecular compounds are represented by chemical formulas, symbolic representations that, at minimum, indicate... 

A considerable body of scientific work has been accomplished in the past to define and characterize point defects. One major reason is that sometimes, the energy of a point defect can be calculated. In others, the charge-compensation within the solid becomes apparent. In many cases, if one deliberately adds an Impurity to a compound to modify its physical properties, the charge-compensation, intrinsic to the defect formed, can be predicted. We are now ready to describe these defects in terms of their energy and to present equations describing their equilibria. One way to do this is to use a "Plane-Net". This is simply a two-dimensional representation which uses symbols to replace the spherical images that we used above to represent the atoms (ions) in the structure. 

Atoms and their symbols were introduced in Chap. 3 and 1. In this chapter, the representation of compounds by their formulas will be developed. The formula for a compound (Sec. 4.3) contains much information of use to the chemist. We will learn how to calculate the number of atoms of each element in a formula unit of a compound. Since atoms are so tiny, we will learn to use large groups of atoms—moles of atoms—to ease our calculations. We will learn to calculate the percent by mass of each element in the compound. We will learn how to calculate the simplest formula from percent composition data, and to calculate molecular formulas from simplest formulas and molecular weights. The procedure for writing formulas from names or from knowledge of the elements involved will be presented in Chaps. 5. ft. and 13. 

Electron dot formulas are useful for deducing the structures of organic molecules, but it is more convenient to use simpler representations—structural or graphic formulas—in which a line is used to denote a shared pair of electrons. Because each pair of electrons shared between two atoms is equivalent to a total bond order of 1, each shared pair can be represented by a line between the symbols of the elements. Unshared electrons on the atoms are usually not shown in this kind of representation. The resulting representations of molecules are called graphic formulas or structural formulas. The structural formulas for the compounds (a) to (e) described in Example 21.1 may be written as follows ... 

A single unstable compound of known composition is placed in the main first volume and is located on the basis of its empirical molecular formula expressed in the Hill system used by Chemical Abstracts (C and H if present, then all other element symbols alphabetically). The use of this indexing basis permits a compound to be located if its structure can be drawn, irrespective of whether a valid name is known for it. A representation of the structure of each compound is given on the third bold title line while the name of the compound appears as the first bold title line. References to the information source are given, followed by a statement of the observed hazard, with any relevant explanation. Cross-reference to similar compounds, often in a group entry, completes the entry. See Trifluoroacetyl nitrite p. 244. 

Figure 3.3. Schematic representation of the diagrams for the alkali metals with a selected number of elements of the p-block. In each box the solid intermediate phases are represented in the positions approximately corresponding to their compositions (long bars congruent melting phases short bars non-congruent phases). In the top part of each box every mark corresponds to a characteristic composition of the liquid phase for which the formation of an associate ( liquid compound ) may be suggested, for instance by the presence of an extremum in the trend of some property of the liquid phase. The symbol 2 L shown for certain ranges of compositions in a few diagrams indicates the presence of a miscibility gap in the liquid state, that is two liquid phases.

As a starting point in the description of the solid intermetallic phases it is useful to recall that their identification and classification requires information about their chemical composition and structure. To be consistent with other fields of descriptive chemistry, this information should be included in specific chemical and structural formulae built up according to well-defined rules. This task, however, in the specific domain of the intermetallic phases, or more generally in the area of solid-state chemistry, is much more complicated than for other chemical compounds. This complexity is related both to the chemical characteristics (formation of variable composition phases) and to the structural properties, since the intermetallic compounds are generally non-molecular in nature, while the conventional chemical symbolism has been mainly developed for the representation of molecular units. As a consequence there is no complete, or generally accepted, method of representing the formulae of intermetallic compounds. 

Figure 1.1 Simple representation of a metabolic pathway. Compound B is the product of the first reaction and the substrate for the second reaction, and so on. Capital symbols represent metabolic intermediates and lower case letters with the suffix ase represent enzymes...

Fig. 8. Representation of a support vector machine. There are three different compounds in this simplified SVM representation. The plus (+) symbols represent active, the minus (-) symbols represent nonactive, and the question mark ( ) symbol represents undetermined compounds. The solid line in the hyperplane and the dotted lines represent the maximum margin as defined by the support vectors.

Figure 22. Schematic representation of the energy transfer processes in the decanuclear compounds. For the graphic symbols, see caption to Figure 21.

Tins symbols employed in chemical formula to-day are, with a few alterations and additions, those used by Berzelius. The formula of simple compounds were represented by writing the symbols of the elements contained in the compound side by side, and this simple representation served for some time. The formulae used, however, did not denote the proportion of the atoms of one kind to that of another kind, and numerals were therefore introduced to denote the number of each kind of atoms in the molecule. This arose naturally when it was found that more than one compound might contain the same elements, and that the different properties of the compounds were due to the proportion of the elements present in the molecule as, for example, the two compounds of carbon and oxygen, carbon monoxide and carbon dioxide. 

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