The Chemical Bond
The States of Matter
|Let us review some of the essential characteristics of the water molecule. The H-O-H bond angle in water, 104.5Ko, reflects tetrahedral orbital hybridization, but with nonbonded pair repulsions causing the bond angle to be squeezed to a bit less than the perfect 109.5o tetrahedral angle. (This bond angle is a favorite MCAT fact.)|
The geometry of water combined with the electronegativity difference between oxygen and hydrogen cause water molecules to be quite polar. Being substantially more electronegative than hydrogen (O = 3.5, H = 2.1), the oxygen nucleus draws the electrons it shares with hydrogen in covalent bonds away from the hydrogen nuclei, pulling them in towards itself, producing a region of partial negative charge around oxygen and partial positive charge around the hydrogen. Water is a polar molecule with a significant dielectric constant.
The type of intermolecular force between water molecules is strong hydrogen bonding, with the positive pole, the nearly exposed hydrogen nuclei, of one water molecule being attracted to the negative oxygen pole of others. The polarity of water and its proclivity to engage in hydrogen bonding are responsible for many important properties such as those depending on its high cohesiveness (high surface tension and high boiling point relative to molecular weight), and for its capacity to dissolve electrolytes.