Covalent compounds physical properties

Properties of Ionic and Covalent Compounds Physical State... 

Physical Properties. The absorption of x-rays by iodine has been studied and the iodine crystal stmcture deterrnined (12,13). Iodine crystallizes in the orthorhombic system and has a unit cell of eight atoms arranged as a symmetrical bipyramid. The cell constants at 18°C (14) are given in Table 1, along with other physical properties. Prom the interatomic distances of many iodine compounds, the calculated effective radius of the covalently bound iodine atom is 184 pm (15). 

Arsenic Halides. Arsenic forms a complete series of trihaUdes, but arsenic pentafluoride is the only well-known simple pentahaUde. AH of the arsenic haUdes, the physical properties of which are given in Table 2, are covalent compounds that hydrolyze in the presence of water. The trihaUdes form pyramidal molecules similar to the trivalent phosphoms analogues and may be prepared by direct combination of the elements. 

A useful property of liquids is their ability to dissolve gases, other liquids and solids. The solutions produced may be end-products, e.g. carbonated drinks, paints, disinfectants or the process itself may serve a useful function, e.g. pickling of metals, removal of pollutant gas from air by absorption (Chapter 17), leaching of a constituent from bulk solid. Clearly a solution s properties can differ significantly from the individual constituents. Solvents are covalent compounds in which molecules are much closer together than in a gas and the intermolecular forces are therefore relatively strong. When the molecules of a covalent solute are physically and chemically similar to those of a liquid solvent the intermolecular forces of each are the same and the solute and solvent will usually mix readily with each other. The quantity of solute in solvent is often expressed as a concentration, e.g. in grams/litre. 

The degree of polarity has considerable influence on the physical properties of covalent compounds and it can also affect chemical reactivity. The melting point (mp) and boiling point (bp) are higher in ionic substances due to the strong nature of the interionic forces, whereas the covalent compounds have lower values due to the weak nature of intermolecular forces. 

Ionic bonds may be fully as strong as covalent bonds, so that properties such as hardness, solubility, melting point, ionization in solution, and chemical character are not especially valuable criteria as a rule. Sometimes comparison of properties with those of compounds of known bond type permits reasonably certain conclusions to be drawn. Thus the similarity in physical properties as well as in atomic arrangement of SiC, AIN, and diamond suggests that all three substances contain covalent bonds. PbS is like FeS2, MoS2, etc. in properties rather than like CaS, so that it is improbable that PbS is an ionic substance. 

The Niccolite Structure. The substances which crystallize with the niccolite structure (B8) are compounds of transition elements with S, Se, Te, As, Sb, Bi, or Sn. The physical properties of the substances indicate that the crystals are not ionic, and this is substantiated by the lack of agreement with the structural rules for ionic crystals. Thus each metal atom is surrounded by an octahedron of non-metal atoms but these octahedra share faces, and the edges of the shared faces are longer than other edges (rather than shorter, as in ionic crystals). Hence we conclude that the bonds are covalent, with probably some metallic character also. 

Methods can be based on some preconceived concept of bonding, with ionic and covalent extremes, or on pattern recognition based on the periodic table. Miscellaneous methods of limited applicability link bond strength with other physical properties. The a priori calculation of heats of formation by wave mechanics is possible in theory. In practice, the most widely applied method incorporates experimental data to derive atom or bond parameters which can then be used for calculations on closely related compounds. 

There are several methods you can use to predict the type of bond in an unknown substance. For example, you can consider the substance s physical properties. In contrast to ionic solids, covalent (molecular) compounds typically have the following properties ... 

In this section, you have used Lewis structures to represent bonding in ionic and covalent compounds, and have applied the quantum mechanical theory of the atom to enhance your understanding of bonding. All chemical bonds—whether their predominant character is ionic, covalent, or between the two—result from the atomic structure and properties of the bonding atoms. In the next section, you will learn how the positions of atoms in a compound, and the arrangement of the bonding and lone pairs of electrons, produce molecules with characteristic shapes. These shapes, and the forces that arise from them, are intimately linked to the physical properties of substances, as you will see in the final section of the chapter. 

With this vast variety, it is usually possible to tailor the solvents to specific chemical reactions. Ionic liquids have a range of physical chemical properties that can be tuned with a precision that is hard to imagine for a given reaction. Ionic liquids are good solvents for a wide range of inorganic, organic, and polymeric materials. There are ionic liquids that will dissolve covalent compounds. Ionic liquids can lead to process intensification. 

Polarity is a physical property of a compound, which relates other physical properties, e.g. melting and boiling points, solubility and intermolecular interactions between molecules. Generally, there is a direct correlation between the polarity of a molecule and the number and types of polar or nonpolar covalent bond that are present. In a few cases, a molecule having polar bonds, but in a symmetrical arrangement, may give rise to a nonpolar molecule, e.g. carbon dioxide (CO2). 

Many textbooks still state that organic lithium compounds have appreciable covalent character. This misconception arises from physical properties such as relatively low melting points and solubility in hydrocarbons or other nonpolar solvents. It is true that these properties... 

From the foregoing you may anticipate that the chemistry of carbon compounds will be largely the chemistry of covalent compounds and will not at all resemble the chemistry of inorganic salts such as sodium chloride. You also may anticipate that the major differences in chemical and physical properties of organic compounds will arise from the nature of the other elements bonded to carbon. Thus methane is not expected to, nor does it have, the same chemistry as other one-carbon compounds such as methyllithium, CH3Li, or methyl fluoride, CH3F. 

Sometimes it is possible to write more than one structural formula to represent a molecular formula. The structural formula of a compound shows how the atoms are joined together by the covalent bonds. For example, there are two different compounds with the molecular formula C4H10. The structural formulae of these two substances along with their names and physical properties are shown in Figure 14.4. 

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