Lewis Structures and Molecular Geometry

Drawing Lewis Structures

A recommended procedure : 1. Count the total number of valence electrons

(VSE) for the atoms (based on the column of the atom in the periodic table) and charge.

(add one electron for each negative charge; subtract one electron for each positive charge). HCN =

1+ 4+ 5 -0 = 10 VSE = 5 VSE pairs C2H6O =

2(4) + 6(1) + 6(1) -0 = 20 VSE = 10 VSE pairs CO3

2- = 4 + 3(6) + 2 = 24 VSE = 12 VSE pairs

Drawing Lewis Structures

2. Draw the framework with single bonds. Some knowledge of the way the atoms are connected may be required.

H C N

3. Complete octets around all the atoms.

H C N

Exceptions: H only has one pair of electrons, Be only has two and B has only three pairs.

Drawing Lewis Structures

4. Count the number of pairs of electrons depicted. If this number is less than the total, add electrons to the central atom until the total number of electrons is correct.

H C N

There are 7 pairs in the drawing and there should only be five.

Unshared pairs are non-bonding electrons

Drawing Lewis Structures

5. If there are extra pairs of electron in the drawing use lone-pairs (non- bonding pairs) to make double or triple bonds and reduce the total number of electron pairs.

H C N H C N

H C N H C N

Two unshared pairs = one bonded pair

Drawing Lewis Structures

Draw the Lewis structure for PCl51. VSE = P (5) and Cl (7x5) = 40 e-, or 20

pairs of electrons.

2. Place P in the center because it is a singleton, and the chlorines around it.

3. Give each Cl an octet and count the pairs.

4. P has more than four pairs called an expanded octet.

Phosphorus Pentachloride

Review:

Lewis structures are a 2 dimensional picture of the arrangement of atoms in a molecule.

The total pairs of electrons must be correct in your final drawing.

Each atom must have four pairs (an octet) of electrons, except H, Be and B.

Some atoms may have more than four pairs, like the second row p orbitals.

Lewis Structures Practice

Draw Lewis Structure for the following compounds.

1. CO2 7. NO3-

2. H2O 8. SO2

3. CH4 9. NH3

4. BeCl2 10. NH4+

5. PO43-

6. PCl6-