4. Electron Sharing and      Covalent Bonds  
     Oxygen and Water

The formation of a water molecule, H-O-H or H2O, is shown at right. The two lone pairs repel one another, and each repels the bonding pairs more than the bonding pairs themselves repel one another. The H-O-H angle therefore is squeezed down from an ideal tetrahedral angle of 109.5 degrees to 105 degrees. Oxygen is more electronegative than nitrogen, so each O-H bond has 33% ionic character, whereas the N-H bond in ammonia has only 27% ionic character. This plus the presence of two lone electron pairs on O increase the dipole moment of the molecule as a whole to 1.85 D, to be compared with 1.47 D for ammonia. Each hydrogen atom in water has a partial positive charge, and the oxygen atom has a partial negative charge. An H20 molecule is a miniature dipole.

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