Chemical bonding molecular compounds

Ionic compounds result from the combination of a positive ion known as a cation and a negative ion called an anion. Salt is an ionic compound in which sodium cations and chloride anions chemically combined. Molecular compounds contain discrete molecular units. Molecular units or molecules are the smallest unit of a molecular compound. Atoms in a compound are held together by covalent bonds. Bonds dictate how atoms are held together in a compound or molecule, but for now, just think of ionic compounds as compounds composed of ions, and molecular compounds as compounds composed of molecules. Sugar, water, and carbon dioxide are examples of molecular compounds. 

Modern chemical synthesis may be said to have begun with chemists understanding of the nature of the chemical bond. The properties of matter on a molecular level—the kinds of chemical bonds in compounds, the shapes of molecules, the effects of shape on polarity and other properties—determine the properties of matter on the macroscopic, or observable, level. As chemists and other materials scientists have learned increasingly more about the properties of elements and compounds, their ability to synthesize novel and useful substances not... 

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... 

Boranes are typical species with electron-deficient bonds, where a chemical bond has more centers than electrons. The smallest molecule showing this property is diborane. Each of the two B-H-B bonds (shown in Figure 2-60a) contains only two electrons, while the molecular orbital extends over three atoms. A correct representation has to represent the delocalization of the two electrons over three atom centers as shown in Figure 2-60b. Figure 2-60c shows another type of electron-deficient bond. In boron cage compounds, boron-boron bonds share their electron pair with the unoccupied atom orbital of a third boron atom [86]. These types of bonds cannot be accommodated in a single VB model of two-electron/ two-centered bonds. 

A is a parameter that can be varied to give the correct amount of ionic character. Another way to view the valence bond picture is that the incorporation of ionic character corrects the overemphasis that the valence bond treatment places on electron correlation. The molecular orbital wavefimction underestimates electron correlation and requires methods such as configuration interaction to correct for it. Although the presence of ionic structures in species such as H2 appears coimterintuitive to many chemists, such species are widely used to explain certain other phenomena such as the ortho/para or meta directing properties of substituted benzene compounds imder electrophilic attack. Moverover, it has been shown that the ionic structures correspond to the deformation of the atomic orbitals when daey are involved in chemical bonds. 

At the molecular level, electric dipole moments are important because they give information about the charge distribution in a molecule. Examination of the experimental data for a few simple compounds reveals that the electric dipole moment is also a property associated with chemical bonds and their polarity. The... 

Because of their cyclic structures, cycloalkanes have two faces as viewed edge-on, a "top" face and a "bottom" face. As a result, isomerism is possible in substituted cycloalkanes. For example, there are two different 1,2-dimethyl-cyclopropane isomers, one with the two methyl groups on the same face of the ring and one with the methyls on opposite faces (Figure 4.2). Both isomers are stable compounds, and neither can be converted into the other without breaking and reforming chemical bonds. Make molecular models to prove this to yourself. 

The investigation of PCSs was based upon the theoretical prerequisites of the conjugation concept. This concept, developed on the basis of theoretical analysis and experimental studies of the properties of compounds with low molecular weights, has played an essential role in the progress of our understanding of the nature of the chemical bond, structure, and the reactivity of substances. 

Extensive quantum chemical calculations have been reported for sulfur-rich compounds in the past two decades. These calculations were used to investigate molecular structures and spectroscopic properties, as well as to understand the nature chemical bonding and reaction mechanism. Many high-level ab initio calculations were used for interpretation of experimental data and for providing accurate predictions of molecular structures and thermochemical data where no reliable experimental values are available. In recent years, density functional calculations have been extensively tested and used on many first- and second-row compounds. These proven DFT methods look promising for larger systems because for their computational efficiency. 

In general, there are insufficient data available for quantitative estimates to be made of the hardnesses of intermetallic compounds. However, in some cases trends can be verified. Figure 8.11 illustrates one of these. It indicates that hardnesses and heats of formation tend to be related. In this case for a set of transition metal aluminides. The correlation in this case might have been improved if the heats per molecular volume couls have been plotted, but thr molecular volumes were not available. Nevertheless, the correlation is moderately good indicating that hardness and chemical bond strengths are related as in other compounds. 

Systematic names describe the molecular structure of a compound. A systematic name consists of two parts, . The is the name of a compound having a particular caibon skeleton. The are a list of one or more radical groups connected by chemical bonds to the parent compound. Substituents are arranged in alphabetical order within a compound name. If the parent contains more than one carbon atom, each substiuent name is preceded by a number identifying the carbon atom in the parent compound to which it is connected. 

---