4. Electron Sharing and      Covalent Bonds  
     Double and Triple Bonds

Carbon and the other second-shell nonmetals, except fluorine, have another very special property: they can share more than one electron pair with the same neighbor atom, thereby creating double and triple bonds. Ethane (right) is a two-carbon compound with only single electron-pair bonds: H3C-CH3. Ethylene, H2C=CH2, is a two-carbon compound with a double bond between the carbon atoms (right). Each carbon atom still shares four electron pairs, but with only three neighbors in ethylene instead of four as in ethane. In acetylene, H-CC-H, the carbon atoms are bound together by three electron pairs in a triple bond (right). For reasons that will become apparent in Chapter 9, two carbon atoms cannot share four electron pairs in a quadruple bond.

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