BONDING Part 2 - External Bonds

I. Classification of Molecules: POLAR or NONPOLAR (Read 16.3)

Review of polar bonds and nonpolar bonds

A. Dipoles

Dipole - a molecule that has two electrically charge regions or poles

Nonpolar covalent bond: does not have a dipole

Example: Br2 Br - Br
Electronegativity (EN) =


H2 H - H
EN =


O2 O - O

EN =


Polar covalent bond: does have a dipole

Example: HCl H - Cl
EN =


H - Cl


H - Cl




Example: C and O form CO2

O C O

EN= 3.5 2.5 3.5


O C O




O C O



Example: H and O form H2O


EN = 3.5

O

H H

EN = 2.1 2.1




O

H H


A covalent bond is polar when it joins atoms of different electronegativies, because the atoms share the electrons unequally.



B. Polar Molecules

Molecules with a net imbalance of charge are polar molecules.

In diatomic molecules, such as HCl, with only one bond, the bond polarity itself causes the molecule to be polar.

H - Cl



C. Nonpolar Molecules

H - H


Cl - Cl

In molecules with more than two atoms, both shape and bond polarity determine whether the molecule is polar or nonpolar.

D. Nonpolar molecules with polar sites (NPWPS)


O C O



Cl

Cl C Cl

Cl

Even though the bonds in these molecules are polar, due to the geometry of the bonds, the identical bond polarities are counterbalanced and the molecule has no net dipole. Therefore, the molecules are nonpolar.



Practice Problems: Polar or Nonpolar?

H2O





NH3





BF3





N2






SiF4





HBr





II. External Bonding Between Polar Molecules and NPWPS

A. Dipole - Dipole Interaction

When polar molecules or nonpolar molecules with polar sites are near one another, as in liquids or solids, their partial charges act as tiny electric fields that orient them and thus they are attracted to each other. This is the dipole - dipole interaction.










B. Hydrogen Bonding (Read 17.1 - 17.2)

Hydrogen bonding is a special type of dipole - dipole interaction.

Occurs between molecules that have an H atom bonded to a small, highly electronegative atom with lone pairs of electron: such as N, O, and F.

Example: H2O














III. Properties of water explained by external bonding

Hydrogen bonding is responsible for many of the unique properties of water:

1) Relatively high boiling point

2) High surface tension

3) Low vapor pressure

4) High heat of vaporization

5) High specific heat capacity

6) Ice is less dense than liquid water
























IV. Dissolving Process