Covalent substance

Ionic compounds in solution and in molten state are good conductors of electricity. Melts and solutions of covalent substances are nonconducting. Inorganic substances rarely undergo combustion, whereas (organic) covalent compounds do so readily. 

Covalent substances and ionic precipitates are written in full. 

In molecular covalent compounds, intermolecular forces are very weak in comparison with intramolecular forces. For this reason, most covalent substances with a low molecular mass are gaseous at room temperature. Others, with higher molecular masses may be liquids or solids, though with relatively low melting and boiling points. 

However, in some covalent substances, known as network solids, atoms are bonded together in a way that forms a network structure. 

In polar covalent substances, the molecules have partial positive and negative charges because of the electronegativity differences between the atoms. The molecules are said to possess a dipole. 

A problem with this method is the fact that mineral oil is a mixture of covalent substances (high-molecular-weight hydrocarbons) and its characteristic absorption spectrum will be found superimposed in the spectrum of the solid analyte, as with the solvents used for liquid solutions discussed previously. However, the spectrum is a simple one (Figure 8.24) and often does not cause a significant problem, especially if the solid is not a hydrocarbon. 

It should be noted that in ionic compounds the interionic forces are much stronger than the intermolecular forces in simple covalent substances and so the melting and boiling points are generally higher. 

Generally, they do not dissolve in water. However, water is an excellent solvent and can interact with and dissolve some covalent molecules better than others. Covalent substances are generally soluble in organic solvents. For a further discussion of solubility of substances in organic solvents see Chapters 14 and 15. 

Draw up a table to summarise the properties of the different types of substances you have met in this chapter. Your table should include examples from ionic substances, covalent substances (simple and giant), ceramics and glasses. 

There is an ill-defined boundary between molecular and polymeric covalent substances. It is often possible to recognise discrete molecules in a solid-state structure, but closer scrutiny may reveal intermolecular attractions which are rather stronger than would be consistent with Van der Waals interactions. For example, in crystalline iodine each I atom has as its nearest neighbour another I atom at a distance of 272 pm, a little longer than the I-I distance in the gas-phase molecule (267 pm). However, each I atom has two next-nearest neighbours at 350 and 397 pm. The Van der Waals radius of the I atom is about 215 pm at 430 pm, the optimum balance is struck between the London attraction between two I atoms and their mutual repulsion, in the absence of any other source of bonding. There is therefore some reason to believe that the intermolecular interaction amounts to a degree of polymerisation, and the structure can be viewed as a two-dimensional layer lattice. The shortest I-I distance between layers is 427 pm, consistent with the Van der Waals radius. Elemental iodine behaves in most respects - in its volatility and solubility, for example - as a molecular solid, but it does exhibit incipient metallic properties. 

Here we consider the factors which determine whether a given compound prefers an ionic structure or a covalent one. We may imagine that for any binary compound - e.g. a halide or an oxide - either an ionic or a covalent structure can be envisaged, and these alternatives are in thermochemical competition. Bear in mind that there may be appreciable covalency in ionic substances, and that there may be some ionic contribution to the bonding in covalent substances. Since there is no simple means - short of a rigorous MO treatment - of calculating covalent bond energies, and since quantitative calculations based upon the ionic model are subject to some uncertainties, the question of whether an ionic or a covalent structure is the more favourable thermodynamically cannot be answered in absolute terms. We can, however, rationalise the situation to some extent. 

In Section 1.5, we emphasised the pertinence of the question stable or unstable with respect to what . So far in this chapter, we have sought to rationalise the right to exist of known, stable covalent substances by devising plausible descriptions of their bonding, accounting properly for the available valence electrons and orbitals of the constituent atoms. We now turn to unstable species, with a view to understanding the factors which deny them a right to exist. 

Molecular formulas apply to covalent substances only. 

The barium carbonate is insoluble. Double substitution reactions occur with the formation of an insoluble substance or a covalent substance. Because barium carbonate is not covalent, it must be insoluble. 

Pressure sintering is especially significant for covalent substances exhibiting a low sintering activity under normal pressure (e.g, SiC, Si3N4), With some substances of this kind, hot pressing is the only way of manufacturing a dense compact ceramic product. 

Ionic compounds tend to have higher boiling points than covalent substances do. Both ammonia, NH3, and methane,... 

Ionic substances generally have much higher forces of attraction than covalent substances. Recall that ionic substances are made up of separate ions. Each ion is attracted to all ions of opposite charge. For small ions, these attractions hold the ions tightly in a crystal lattice that can be disrupted only by heating the crystal to very high temperatures. 

For covalent substances, forces that act between molecules are called intermolecular forces. They can be dipole-dipole forces or London dispersion forces. Both forces are short-range and decrease rapidly as molecules get farther apart. Because the forces are effective only when molecules are near each other, they do not have much of an impact on gases. A substance with weak attractive forces will be a gas because there is not enough attractive force to hold molecules together as a liquid or a solid. 

Compare the properties of an ionic substance, NaCl, with those of a nonpolar substance, I2, as shown in Figure 14. The differences in the properties of the substances are related to the differences in the types of forces that act within each substance. Because ionic, polar covalent, and nonpolar covalent substances are different in electron distribution, they are different in the types of attractive forces that they experience. 

Above all, we must recognize that any classification of a compound that we might suggest based on electronic properties must be consistent with the physical properties of ionic and covalent substances described at the beginning of the chapter. For instance, HCl has a rather large electronegativity difference (0.9), and its aqueous solutions conduct electricity. But we know that we cannot view it as an ionic compound because it is a gas, and not a solid, at room temperature. Liquid HCl is a nonconductor. 

The bond energy (B.E.) is the amount of energy necessary to break one mole of bonds in a gaseous covalent substance to form products in the gaseous state at constant temperature and pressure. 

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