4. Electron Sharing and      Covalent Bonds  
     Oxygen and Water

Oxygen has six outer electrons, two lone pairs and two unpaired electrons that are easily available for bonding. When oxygen makes two covalent bonds as in water, H2O, it is surrounded by four electron pairs, two bonding pairs and two lone pairs, in roughly tetrahedral arrangement (see right). This restriction to four electron pairs around the atom, which we have seen for nitrogen and carbon, is mainly a consequence of the small size of the atoms. An oxygen atom does not share more of its outer-shell electrons because there is no room around it for more than four neighbor atoms. In larger atoms with the same six electron outer-shell structure of oxygen, such as sulfur (third shell), selenium, and tellurium, all six of the electrons can be shared, thereby bringing six electron pairs around the central atom. But with the small second-row elements, four pairs is the maximum, and an eight-electron shell is full.

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