IB DP Chemistry: SL复习笔记4.2.4 Molecular Polarity

Assigning polarity to molecules

• There is a difference between bond polarity and molecular polarity 
• To determine whether a molecule is polar, the following things have to be taken into consideration:
  • The polarity of each bond
  • How the bonds are arranged in the molecule
• Some molecules have polar bonds but are overall not polar because the polar bonds in the molecule are arranged in such way that the individual dipole moments cancel each other out

There are four polar covalent bonds in CH₃Cl which do not cancel each other out causing CH₃Cl to be a polar molecule; the overall dipole is towards the electronegative chlorine atom

Though CCl₄ has four polar covalent bonds, the individual dipole moments cancel each other out causing CCl₄ to be a nonpolar molecule

• Further examples of molecules with no net dipole:

Carbon dioxide and boron trifluoride have polar bonds but no net dipole

• Try your hand at this polarity question:

Answer:

The correct option is D.

• • The shapes and polarity of the molecules are as follows:

Although the Al-Br bonds are polar, the trigonal planar molecule is symmetrical so the dipoles cancel out leaving a non-polar molecule