CMB 311 Lecture Notes - Lecture 2: Lone Pair, Hydrogen Bond, Boiling Point
26 Jan 2019
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An unusually high boiling point, compared to other molecules of similar molecular weight and structure, e. g. ammonia (nh3) or methane (ch4). The density of water decreases upon freezing (as you know, ice floats); most substances contract when freezing and have a higher density in the solid state. These properties are due to hydrogen bonding, which is a defining feature of water. This is due to the large difference in relative electronegativity of oxygen (3. 5) and hydrogen (2. 1). Further, because water has two unshared pairs of electrons, the two hydrogen atoms in water are not on opposite sides of the oxygen atom but form two o-h bonds at a 104. 3 angle. The unshared electron pairs plus the two hydrogens form a distorted tetrahedron. An ideal tetrahedron has angles of 109. 5 but water is distorted due to repulsion between the negative charges of the lone pair electrons. Oxygen, 2 hydrogen, and 2 lone pairs (causes distortion)
