Crystalline solids ionic bonding

When crystals with covalent bonds (e.g., AICI3 or TiCy melt, the melt conductivity remains low (e.g., below 0.1 S/m), which implies that the degree of dissociation of the covalent bonds after melting is low. The covalent crystals also differ from the ionic crystals by their much lower melting points. The differences between these two types of crystal are rather pronounced, whereas there are few crystalline solids with intermediate properties. 

These terms are frequently used and may lead to confusion, if used in the wrong context. As a reminder, the valency of an atom is strictly the number of bonds (including o, n and 8) in which it participates in any particular compound. In IF , the iodine atom participates in seven a bonds to the fluorine atoms. The fluorine atoms are individually monovalent. Oxidation states are more useful than valency in describing ionic compounds. In the crystalline solid CaF0, the calcium is best thought of as composed of calcium dications and fluoride anions, Ca2+(F ),. The calcium is in its +2 oxidation state, having lost its valency electrons, and the fluorine atoms are in the -1 oxidation state, both having accepted an elec-... 

The properties of lithium metal are well known, but the properties of its alkyls have until recently received much less attention. The lowest member of the series, methyllithium, is a non-volatile microcrystalline powder insoluble in hydrocarbons. Ethyllithium is a colourless crystalline compound melting at 95°. n-Propyl and n-butyllithium are almost colourless fairly viscous non-volatile oils soluble in hydrocarbons and ethers. These properties are to be compared with those of the corresponding sodium alkyls which are all colourless, non-volatile crystalline solids, insoluble in hydrocarbons. The difference in properties is usually attributed to differences in the type of bond between lithium and sodium alkyls, the former being considered covalent and the latter ionic compounds. Thus Coates (17) distinguishes between two types of compounds ... 

An ionically bonded molecule (NaCl). (a) A sodium atom (Na) can donate the one electron in its valence shell to a chlorine atom (Cl), which has seven electrons in its outermost shell. The resulting ions (Na+ and CP) bond to form the compound sodium chloride (NaCl). The octet rule has been satisfied, (b) The ions that constitute NaCl form a regular crystalline structure in the solid state. 

The oppositely charged Na+ and Cl- ions that result when sodium transfers an electron to chlorine are attracted to one another by electrostatic forces, and we say that they are joined by an ionic bond. The crystalline substance that results is said to be an ionic solid. A visible crystal of sodium chloride does not consist of individual pairs of Na+ and Cl- ions, however. Instead, solid NaCl consists of a vast three-dimensional network of ions in which each Na+ is surrounded by and attracted to many Cl - ions, and each Cl- is surrounded by and attracted to many Na+ ions (Figure 6.7). 

Solids can be characterized as amorphous if their particles are randomly arranged or crystalline if their particles are ordered. Crystalline solids can be further characterized as ionic solids if their particles are ions, molecular solids if their particles are molecules, covalent network solids if they consist of a covalently bonded array of atoms without discrete molecules, or metallic solids if their particles are metal atoms. 

The nature of the bonds between the structural units of crystalline solids impart other physical properties to these solids. Metals are good conductors of electricity because metallic bonds allow a free flow of electrons. Covalent network, molecular, and ionic solids do not conduct electricity because their bonds do not provide for mobile electrons. Remember, however, that ionic solids in a water solution have free electrons and are good conductors of electricity. Metallic solids are malleable and ductile covalent network solids are brittle and hard. These differences in physical properties are caused by the chemical bonds between the units It is all in the bonds ... 

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