O 96 pm 104.5o H H Fig. 1. A Model for the Water Molecule

Fig. 2. Molecules of CH4, NH3 and H2O Lone pair electrons • • • • • • CH4 C–H, 110 pm H-C-H, 109.5o NH3 H–N, 100 pm H-N-H, 107o H2O H–O, 96 pm H-O-H, 104.5o Fig. 2. Molecules of CH4, NH3 and H2O

1 2 3 Fig. 3. The three principle vibration modes of the water molecule, H2O: 1, symmetric stretching, 2, bending, and 3, asymmetric stretching.

Fig. 4. Infrared absorption spectrum of Gas-phase H2O plotted using NIST/EPA Gas-phase infrared database (webbok.nist.gov/chemistry)

H O H H O (a) H H O O H H (b) Fig. 5. Two possible structures of the water dimer, (H2O)2