Covalent compound melting point

In most covalent compounds, the strong covalent bonds link the atoms together into molecules, but the molecules themselves are held together by much weaker forces, hence the low melting points of molecular crystals and their inability to conduct electricity. These weak intermolecular forces are called van der WaaFs forces in general, they increase with increase in size of the molecule. Only... 

Table 14.2 shows that all three elements have remarkably low melting points and boiling points—an indication of the weak metallic bonding, especially notable in mercury. The low heat of atomisation of the latter element compensates to some extent its higher ionisation energies, so that, in practice, all the elements of this group can form cations in aqueous solution or in hydrated salts anhydrous mercuryfll) compounds are generally covalent. 

Metalloid peroxides behave as covalent organic compounds and most ate insensitive to friction and impact but can decompose violentiy if heated rapidly. Most soHd metalloid peroxides have weU-defined melting points and the mote stable Hquid members can be distilled (Table 3). Some... 

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. 

Xe] 4/ 145d,()6s2. (d) Because compound II h as a lower melting point, it is probably more covalent, which is consistent with the fact that the +3 ion is more polarizing. 

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. 

Boron carbide is a non-metallic covalent material with the theoretical stoichiometric formula, B4C. Stoichiometry, however, is rarely achieved and the compound is usually boron rich. It has a rhombohedral structure with a low density and a high melting point. It is extremely hard and has excellent nuclear properties. Its characteristics are summarized in Table 9.2. 

Since these structures are formed by filling the open spaces in the diamond and wurtzite structures, they have high atomic densities. This implies high valence electron densities and therefore considerable stability which is manifested by high melting points and elastic stiffnesses. They behave more like metal-metalloid compounds than like pure metals. That is, like covalent compounds embedded in metals. 

Ionic compounds, as compared to covalent compounds, tend to have greater densities, higher melting and boiling points, and can be soluble in the very polar solvent, water, if the ionic bond is not too strong. 

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. 

Problem 1.2 How do the boiling points, melting points, and solubilities of covalent organic compounds differ from those of salts Account for the differences. 

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... 

Why are the melting points of most ionic compounds far higher than the melting points of most covalent compounds ... 

Look at the comparison between NaCI and HCI, shown in Table 7.2. Sodium chloride, an ionic compound, is a white solid with a melting point of 801°C and a boiling point of 1413°C. Hydrogen chloride, a covalent compound, is a colorless gas with a melting point of — 115°C and a boiling point of —84.9°C. What accounts for such large differences between the properties of ionic compounds and covalent compounds ... 

The binary hydrides are compounds that contain hydrogen and just one other element. Formulas and melting points of the simplest hydrides of the main-group elements are listed in Figure 14.2. Binary hydrides can be classified as ionic, covalent, or metallic. 

In earlier chapters, we saw examples of how the metallic or nonmetallic character of an element affects its chemistry. Metals tend to form ionic compounds with nonmetals, whereas nonmetals tend to form covalent, molecular compounds with one another. Thus, binary metallic hydrides, such as NaH and CaH2, are ionic solids with high melting points, and binary nonmetallic hydrides, such as CH4, NH3, H20, and HF, are covalent, molecular compounds that exist at room temperature as gases or volatile liquids (Section 14.5). 

Look up the melting and boiling points of simple covalent compounds in data books. 

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