√ The Attractive Forces between Molecules Explained with Examples

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Hydrogen bonding
• The hydrogen bond is a special type of dipole-dipole force between molecules that have an H atom bound to a small, highly electronegative atom with lone pair electrons. N, O and F are those atoms that strongly withdraw electron density from H. As a result N-H, O-H and F-H bonds are very polar and H becomes partially positive. The partially positive H of one molecule is attracted to the partially negative lone pair on N, O or F of another molecule. This attraction is called a hydrogen bond.
• The small sizes of N, O or F are vital to H-bond
The small sizes allow these atoms to be strongly electronegative and their bonded H to be highly positive
The small sizes allow the lone pair on N, O or F to come closer to the H
• Other electronegative elements such as chlorine, bromine and sulfur do not form hydrogen bonds. Although H-Cl, H-Br and H-S bonds are polar, the attractive force between the partially positive hydrogen atoms and the lone electron pairs on other chlorine, bromine and sulfur atoms is not as strong as that between H and N, O or F. The chlorine, bromine and sulfur atoms are much larger and the lone pair electrons are not as accessible to the partial positively charged hydrogen atoms.

Summary of the essential requirements for hydrogen bonding
• H atom bonded too strongly electronegative and small N, O or F so that H atom becomes partially positively charged.
• Lone pair of electrons on an N, O or F of another molecule which can attract the partially positive H atom.
Significance and features of H bonding
• The strength of hydrogen bonds is in general about ten times those of dipole-¬dipole forces but about one-tenth those of ionic or covalent bonds.
• Hydrogen bonds are extremely important in biological systems and play a critical role in determining the structure of proteins, For example, DNA molecule consists of two chains. Each is held together by strong covalent bonds, but millions of H bonds link one chain to the other to form a double helix.
• Hydrogen bonding is important in many chemical systems. Boiling and melting points typically rise as molar mass increases, as we can see in the Group 4 hydrides, CH4 through SnH4. However, the first member in each series – NH3, H2O and HF- deviates enormously from this expectation. This is because H-bonds keep these molecules together and additional energy is required to break the H-bonds to separate these molecules to liquid state or gas state.
H-bond and uniqueness of water
• The presence of hydrogen bonding accounts for many of the unique properties of water. For example, the arrangement of water molecules in ice creates a very open structure which causes the density of ice to be less than that of liquid water.
• When ice melts, the regular lattice breaks up and the water molecules can pack more closely to form a liquid of somewhat higher density. Without hydrogen bonding, ice would sink to the bottom of oceans and lakes, a process that in cold climates would cause the death of fish and other aquatic life.