4. Electron Sharing and      Covalent Bonds  
     Oxygen and Water

One oxygen atom can satisfy all of its electron-sharing needs by using both unpaired electrons in a double bond to another oxygen atom, as shown in the figure on the right. Oxygen therefore occurs as a diatomic gas, 0=0 or O2, similar to N2 gas but totally unlike the infinite solid structures of carbon in diamond or graphite. Solid boron and solid carbon are held together by covalent bonds in a three-dimensional network, and melt only at very high temperatures: 2037C for boron and 3500C for graphite. Molecules of solid N2 and O2 are held together only by van der Waals forces, and therefore melt at a low -210C and -219C, respectively.

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