Molecular Shapes and Molecular Polarity

• Recall that bond polarity is a measure of how equally the electrons in a bond are shared between two atoms. As the difference in electronegativity between two atoms increases, so does bond polarity.

• In molecules containing two atoms, a dipole exists if there is a difference in electronegativity.

• For a molecule with more than two atoms, the dipole moment depends on both the polarities of the individual bonds and the geometry of the molecule.

• Consider linear CO2

• Each C=O bond is polar, but since the bonds are identical, the bond dipoles are equal in magnitude. The overall dipole moment = 0.

• Bond dipoles and dipole moments are vector quantities, which means they have magnitude and direction. The overall dipole moment of a molecule is the sum of its bond dipoles.

• Consider the bent molecule, water, with two polar bonds.

• Both bonds are identical so the bond dipoles are equal, but since the molecule is bent, the bonds do not directly oppose each other. Therefore, the bond dipoles do not cancel each other out.

• The water molecule has a non zero dipole moment so it is polar. The oxygen carries a partial negative charge and the hydrogen atoms each carry partial positive charges.

Comparing Models

Drawing Lewis Structures1. Arrange the element symbols.

• Central atoms are generally those with the highest bonding capacity.

• Carbon atoms are always central atoms

• Hydrogen atoms are always peripheral atoms

2. Add up the number of valence electrons from all atoms. • Add one electron for each negative charge and subtract one for each

positive charge.

3. Draw a skeleton structure with atoms attached by single bonds.

4. Complete the octets of peripheral atoms. Hydrogen will not have any lone pairs!

5. Place extra electrons on the central atom.

6. If the central atom doesn’t have an octet, try forming multiple bonds by moving lone pairs.

Structural Formula7. From the Lewis structure, remove dots

representing lone pairs8. Replace bond dots with a dash

WaterHydrogen (1) + Hydrogen (1) + Oxygen (6) = 8

SAMPLE PROBLEM A) Writing Lewis Structures for Molecules with One Central Atom

SOLUTION:

PROBLEM: Write a Lewis structure for CCl2F2, one of the compounds responsible for the depletion of stratospheric ozone.

Step 1: Carbon has the highest bonding capacity and is the central atom. The other atoms are placed around it.

C

Steps 2-4: C has 4 valence e-, Cl and F each have 7. The

sum is 4 + 4(7) = 32 valence e-.

Cl

Cl F

F

C

Cl

Cl F

FMake bonds and fill in remaining valence electrons placing 8e- around each atom.

:

::

::

:

:

::

: ::

Ammonia (NH3)

Nitrogen (5) and Hydrogen 3(1) = 8

Drawing Lewis Structures

COCl2 24 ve’s

HOCl 14 ve’s

ClO3 26 ve’s

CH3OH 14 ve’s

Cl C Cl

O

H O Cl

O Cl O

O

H C O H

H

H

SAMPLE PROBLEM B) Writing Lewis Structure for Molecules with More than One Central Atom

PROBLEM: Write the Lewis structure for methanol (molecular formula CH4O), an important industrial alcohol that is being used as a gasoline alternative in car engines.

SOLUTION: Hydrogen can have only one bond so C and O must be next to each other with H filling in the bonds.

There are 4(1) + 4 + 6 = 14 valence e-.

C has 4 bonds and O has 2. O has 2 pair of nonbonding e-.

C O H

H

H

H

::

Multiple Bonds

• So far we have only looked at single bonds. The sharing of a pair of electrons constitutes a single bond.

• In many molecules atoms attain complete octets by sharing more than one pair of electrons.

• When two electron pairs (4 electrons) are shared, two lines are drawn to represent the double bond.

• A triple bond corresponds to sharing of three pairs of electrons (6 electrons).

SAMPLE PROBLEM C) Writing Lewis Structures for Molecules with Multiple Bonds.

PLAN:

SOLUTION:

PROBLEM: Write Lewis structures for the following:

(a) Ethylene (C2H4), the most important reactant in the manufacture of polymers

(b) Nitrogen (N2), the most abundant atmospheric gas

For molecules with multiple bonds, there is a Step 5 which follows the other steps in Lewis structure construction. If a central atom does not have 8e-, an octet, then e- can be moved in to form a multiple bond.

(a) There are 2(4) + 4(1) = 12 valence e-. H can have only one bond per atom.

CCH

H H

H

:

CCH

H H

H

(b) N2 has 2(5) = 10 valence e-. Therefore a triple bond is required to make the octet around each N.

N

:

N

:

. .

..

N

:

N

:

. . N

:

N

:

Polyatomic Ions

• Many compounds contain a combination of covalent and ionic bonds.

• E.g. NaOH

• OH- is a polyatomic ion with a covalent bond between O and H

• Ionic bond between Na+ and OH-

Coordinate Covalent BondsA covalent bond in which both of the shared electrons come

from the same atom.

E.g. NH3 (ammonia) and H+ (hydrogen ion) to form NH4

(ammonium)