Polar Covalent Solids

Strongly for the ionic crystals, yet the bulk modulus for the alkali halides varies as d. The cl trend for the bulk modulus will show up in the study of simple metals, and in terms of the pseudopotentials that will be used in the study of simple metals, d -dependence takes on a particularly fundamental role. In Problem 15-3, the simple metal theory is used to give a good account of the bulk modulus in C, Si, and Gc. It should be noted also that the simple metal theory docs not give a good account of cohesive energy itself there is much cancellation between terms for that property, and there are important contributions (for example, that do not vary as 

We see that the predictions are very good, except for the noble-metal halides, which arc much more strongly bound than is predicted we associate the discrepancy with the noble-metal d bands. We also see that the predictions are very crude in the diamond row, as has happened with other properties. 

The cohesive energy per bond is a quantity of sufficient fundamental importance that we have included in Table 7-3 also the experimental values for the tetrahedral solids other than those isoelectronic with C, Si, Ge, and Sn. We should 

Hybrid covalent and hybrid polar energies, in eV. The hybrid polarity a can be compared with the polarity a, based upon p states and used to describe dielectric properties. 

Cohesive energy per bond, in eV (multiplying by 92.2 gives the value in kilocalories per mole for the polar semiconductors). 

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The solubility of a given solute in a particular solvent depends on a number of factors. One generalization which can be used for determining solubility is like dissolves like. This means that the more similar the polarity of a solute is to the polarity of the solvent, the more likely the two will form a homogeneous solution. A polar solvent, such as water, will dissolve a polar compound an ionic salt like common table salt, NaCl, will dissolve in water a polar covalent solid like table sugar, sucrose, will dissolve in water. Nonpolar solvents such as naphtha or turpentine will dissolve nonpolar material, such as grease or oil. On the other hand, oil and water do not mix because of their different polar characteristics. 

For polar covalent solids, sp hybrids of the form of Eq. (3-1) can be constructed on each of the atom types present and oriented in the directions of its nearest neighbors. The hybrid energies will be different we call the lesser energy and the greater energy i-, and, in direct analogy with Eq. (1-32), define a hybrid polar energy proportional to their difference ... 

A special case of closed-shell configurations are the ions of zero charge, the inert gases themselves. The inert-gas solids may not be of great intrinsic interest, but they provide the best starting point for the understanding of ionic solids, so we discuss them first. We shall then see how the ionic solids can be understood in terms of transfer of protons between nuclei, much as we understood the polar covalent solids in terms of transfer of protons between the nuclei of the homopolar solids. 

Let us also make an application of the chemical grip to polar covalent solids. This corresponds to beginning in the limit of unit polarity, V [V + = I,... 

With few exceptions, the metal oxides are ionic solids and react with water to form aqueous ions, the nonmetal oxides are network covalent solids that react with water to make covalent compounds, and the amphoteric oxides of the metalloids form oligomeric polar-covalent solids. Similar relationships hold for the hydrides and fluorides of each element, with the metal forming an ionic solid and the non-metal forming a network covalent solid, although the actual demarcation line varies somewhat depending on the anion. 

Chlorine gas is very reactive, and causes horrific bums to the eyes and throat see p. 243. The two atoms are held together by means of a single, non-polar covalent bond. CI2 has a yellow-green colour and, for a gas, is relatively dense at s.t.p. Conversely, table salt (sodium chloride) is an ionic solid comprising Na+ and Cl- ions, held together in a three-dimensional array. What is the reason for their differences in behaviour ... 

FIG. 3 The vapor phase water dimer structure. Polar covalent bonds are shown as solid lines and the hydrogen bond as a dashed line (adapted from Ludwig, 2001). 

In this investigation, you will study the properties of five different types of solids non-polar covalent, polar covalent, ionic, network, and metallic. You will be asked to identify each substance as one of the five types. In some cases, this will involve making inferences and drawing on past knowledge and experience. In others, this may involve process-of-elimination. The emphasis is on the skills and understandings you use to make your decisions. Later, you will be able to assess the validity of your decisions. 

Based on what you know about bonding, classify each solid as non-polar covalent, polar covalent, ionic, network, or metallic. Give reasons to support your decision. 

Polar framework compounds. These are compounds where no individual molecules exist, and range from ionic compounds like sodium chloride, through part-ionic, part-covalent compounds like aluminum oxide, to polar covalent framework solids like silicon dioxide. 

Comparing polarity between components is often a good way to predict solubility, regardless of whether those components are liquid, solid, or gas. Why is polarity such a good predictor Because polarity is central to the tournament of forces that underlies solubility. So solids held together by ionic bonds (the most polar type of bond) or polar covalent bonds tend to dissolve well in polar solvents, like water. 

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