2.2.6 Molecular Polarity

Similar to bond polarity, but describes the electron distribution throughout the whole molecule.

Recap: A dipole moment ( $\mu$) happens when a molecule is polar ( partial negative charge on one end and partial positive charge on the other ).

• The value of the dipole moment is in the unit debye, D.

• Polarity is an important trait in molecules as it gives rise to multiple characteristics:

  • Volatility

  • Solubility

  • Boiling point

• Molecular polarity depends on both bond Polarity and molecular geometry.

examples

• Molecules are polar when their bond dipoles do not cancel each other out.

  • May occur due to the geometry of molecule or the bonds have different magnitudes for the dipole moment.

  • AS seen from water and trichloromethane, the polar bonds do not cancel each other--> resulting in a net dipole charge.

• Molecuels are not-polar when their bond dipoles cancel each other out

  • Boron triflouride

  • Carbon dioxide

• Molcules are non-polr when all their bonds are non-polar

  • Hydrocarbons ( hydrogen + carbons ), like Ethane contain two types of bonds --> carbon-carbon bonds, and carbon-hydrogen bonds

    • Carbon-carbon cancels each other out, and carbon-hydrogen bonds are virtually non-polar because of the such small electronegativity difference.

    • The tetrahedral geometry around each carbon atom results iin a net dipole of zero, or very close to zero.

    • $\therefore$ Hydrocarbons are usually non-polar

• Small Molecules were discussed so far--> What about long ones?

• Due to its length, they can have a region of polar and non-polar.

  • It leads to applications in emulsifiers, soaps, and detergents, as well as the phospholipid bilayer in cell membranes.

Teacher notes:

if seen lone pairs in central atom it is a polar compound since the lone pairs will bend the molecular geometry.

OR

**If two different atoms bond to the central atom then its .......

POLAR