Network covalent compounds

Chemical bonding is different in metals than it is in ionic, molecular, or covalent-network compounds. This difference is reflected in the unique properties of metals. They are excellent electrical conductors in the solid state—much better conductors than even molten ionio oompounds. This property is due to the highly mobile valence electrons of the atoms that make up a metal. Such mobility is not possible in molecular compounds, in which valence electrons are localized in electron-pair bonds between neutral atoms. Nor is it possible in solid ionic compounds, in whioh electrons are bound to individual ions that are held in place in crystal structures. 

Some covalent compounds do not consist of individual molecules. Instead, each atom is joined to aU its neighbors in a covalently bonded, three-dimensional network. There are no distinct units in these compounds, just as there are no such units in ionic compounds. The subscripts in a formula for a covalent-network compound indicate the smallest whole-number ratio of the atoms in the compound. Naming such compounds is similar to naming molecular compounds. Some common examples are given below. 

The vast range in elemental properties, from those of metallic elanents such as thallium and lead that have very low electronegativities of 1.8 and 1.9 (respectively) to those of the nonmetalUc elements such as oxygen and fluorine that have the highest electronegativities of 3.5 and 4.0 (respectively), results in the great chemical diversity of the elements in the p block. These elements include metals, alloys, simple covalent compounds, enormous covalent network compounds, simple binary ionic compounds, and complex chain and layered ionic compounds. 

Homonuclear aggregates of phosphorus atoms exist in many forms discrete molecules, covalent networks in crystals, polyphosphide anions, and phosphorus fragments in molecular compounds. 

The correct answer is (B). NH3 has the weakest intermolecular forces of the other molecules. Diamond exists in a covalent network bond, sodium acetate is an ionic compound, and glycerine contains several C O and O H bonds (which allow hydrogen bonding). Silver has metallic bonds while ammonia, NH3, is only held together by fairly weak hydrogen bonds (the N-H bond is not very polar). 

The type of attractive forces within solids depends on the identity of the unit particle and the chemical bonds it can form. The forces between atoms in a covalent network solid (such as carbon in diamond) are covalent bonds. These bonds result when at least one pair of electrons is shared by two atoms. The forces between atoms within metallic elements (such as iron) are metallic bonds. Electrostatic attractions—also called ionic bonds—are the forces between ions, atoms which have lost one or more electrons to become positively charged ions or which have gained one or more electrons to become negatively charged ions (such as those found in NaCI). Ionic compounds are often known as salts. Covalent, metallic, and ionic bonds are strong chemical bonds. 

Clathrates are host-guest complexes in which a crystalline cage of the host compound holds the guest molecule by weak intermolecular forces. Often, the cavities of the guest molecule are formed by a network of hydrogen bonds between covalently bound compounds. Powell [126] names them clathrates from the Latin word clathratus , which means enclosed. It is interesting to note that the lattice structure of the host in the clathrate is not its normal crystalline form the former becomes thermodynamically more stable than the latter only by the formation of the host-guest complex. 

Elemental semicondudDis, like Si and Ge, as well as compound semiconductors, like GaAs, InP, and CdTe, are important examples of covalent-network solids. In a semiconductor the filled bonding molecular orbitals make up the valence band, while the empty antibonding molecular orbitals make up the conduction band. The thence and conduction bands arc separated by an energy that is referred to as the band gap. The size of the band gap increases as the bond distance decreases, and as the difference in electronegativity between the two elements increases. 

Manganese silicide has the empirical formula MnSi and melts at 1280 C. It is insoluble in water but does dissolve in aqueous HF. (a) What type of compound do you expect MnSi to be metallic, molecular, covalent-network, or ionic (b) Write a likely balanced chemical equation for the reaction of MnSi with concentrated aqueous HF. 

Indicate the type of solid (molecular, metallic, ionic, or covalent-network) for each compound (a) CaC03, (b) Pt, (c) Zr02 (melting point, 2677 °C), (d) table sugar (C12H22O11),... 

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