Molecule

• molecule (פְּרֻדָּה, מוֹלֵקוּלָה)
  • “a group of two or more atoms that are held together by chemical bonds” (Wikipedia)
  • nonpolar molecule: “A molecule that has only nonpolar bonds or in which the bond dipoles cancel.” (Timberlake, 2017)
    • examples:
      • $\mathrm{CO}{}_2$ has polar bonds the bond dipoles are equal in magnitude but opposite in direction (linear), canceling each other out
      • homonuclear diatomic molecules: $\mathrm{N}{}_2,\mathrm{H}{}_2,\mathrm{O}{}_2$
  • polar molecule “A molecule containing bond dipoles that do not cancel.” (Timberlake, 2017)
    • examples: $\mathrm{H}{}_2\mathrm{O}$
  • polarity
    • If the molecule does not contain polar bonds, it is nonpolar.
    • If the molecule is diatomic and its bond is polar, it is polar.
    • If the molecule contains two or more polar bonds, it may or may not be polar. The result depends on whether the bond dipoles cancel due to molecular geometry. We use the following key rules:
      • Let $n$ be the number of lone pairs on the central atom:
        • if $n=0$, and all terminal atoms are the same, then it is nonpolar. (e.g. $\mathrm{CO}{}_2$, $\mathrm{CH}{}_4$)
        • if $n=1$, then it is polar. (e.g. $\mathrm{NH}{}_3$, $\mathrm{SO}{}_2$)
        • if $n\ge 2$, then it is usually polar. (e.g. $\mathrm{H}{}_2\mathrm{O}$, $\mathrm{H}{}_2\mathrm{S}$ are polar, but $\mathrm{XeF}{}_4$ is not)
      • Most hydrocarbons are nonpolar.
  • diatomic molecule
    • ”molecule composed of only two atoms, of the same or different chemical elements” (Wikipedia)
  • homonuclear molecule (molecule composed of only one element)

Properties

• vapor pressure
• molecular dipole moment $\mu$
• molar mass

References

• Timberlake, Karen C. (2017). Chemistry: An Introduction to General, Organic, and Biological Chemistry. Pearson.