The uneven distribution of electrons throughout a molecule (polarity) is determined by bond polarity and molecular shape. In addition, intermolecular forces act between molecules.
Basic Knowledge
Bond polarity: In a bond between two atoms with different electronegativities, the side attracting the electrons becomes δ-, and the other becomes δ+.
Molecular polarity: Consider both bond polarity and molecular shape (symmetry).
・Symmetrical molecules (linear CO2, tetrahedral CH4): nonpolar molecules (polarities cancel)
・Asymmetrical molecules (bent H2O, trigonal pyramidal NH3): polar molecules
Intermolecular forces are weak forces between molecules, and there are three types:
① Van der Waals forces: Weak attractions acting between all molecules, including nonpolar molecules through instantaneous electron distribution. They become stronger as molecular mass increases.
② Dipole–dipole interactions: δ+ and δ- regions of polar molecules attract one another.
③ Hydrogen bonds: A specially strong intermolecular force in which H directly bonded to F, O, or N attracts a lone pair on F, O, or N in another molecule. They occur in H2O, HF, and NH3.
Polar molecule（A molecule with overall positive and negative regions caused by uneven electron distribution. H2O, HCl）
Nonpolar molecule（A symmetrical molecule in which polarities cancel. CO2, CH4, N2）
Intermolecular force（A general term for weak attractions acting between molecules）
Van der Waals force（A weak force acting between all molecules. It becomes stronger as molecular mass increases）
Hydrogen bond（A strong intermolecular force in which H bonded to F, O, or N attracts another atom）
Molecular crystal（A crystal in which molecules are arranged by intermolecular forces. Ice, iodine, dry ice）
Further Study (Background and Significance)
All of water’s unusual properties can be explained by hydrogen bonds.
・Water has an unusually high boiling point of 100℃ (although the boiling points of H2S, H2Se, and H2Te should increase down the group, water alone is exceptionally high).
・Ice has a lower density than water because its tetrahedral hydrogen-bond network creates spaces. This allows ice to float and pond fish to survive winter.
・It has a high specific heat, so it is difficult to heat and cool. This helps moderate Earth’s climate.
・It has high surface tension, which makes water droplets round and allows water striders to stand on the surface.
Hydrogen bonds between bases also maintain DNA’s double helix (A=T, G≡C). The three-dimensional structures of proteins are stabilized by hydrogen bonds, making intermolecular forces a core concept in life science.
Molecular crystals (ice, iodine, and dry ice) are held together by weak intermolecular forces, so they have low melting and boiling points, are soft, and readily sublime.
- Molecular polarity = bond polarity + shape (symmetry)
- CO2 is nonpolar (symmetrical); H2O is polar (bent)
- Intermolecular forces = van der Waals forces + dipole interactions + hydrogen bonds
- Hydrogen bonds occur with F-H, O-H, and N-H
- Water’s boiling point, ice density, and specific heat are all affected by hydrogen bonds
- The DNA double helix also involves hydrogen bonds (A-T, G-C)
- Molecular crystals have low melting points, are soft, and readily sublime
Points to Note (Common Confusions)
① Bond polarity (between atoms) and molecular polarity (overall) are different. ② Even molecules with similar molecular masses have higher boiling points when hydrogen bonds are present (H2O > H2S, for example). ③ A hydrogen bond is a type of intermolecular force, not a covalent bond. Its energy is about one-tenth that of a covalent bond. ④ Molecular crystals, ionic crystals, and covalent-network crystals are different concepts.
Practice
- Although CO2 is nonpolar, its C=O bonds are polar. Explain why.
- Using the term intermolecular force, explain why water’s boiling point is unusually higher than that of H2S.
- What is the especially strong intermolecular force that occurs with F-H, O-H, and N-H?