Honors Chemistry

Lewis Structures and VSEPR Theory

Lewis Structures

• A Lewis structure shows the distribution of outer (valence) electrons in an atom, molecule, or polyatomic ion.

• Unshared electrons are shown as dots• Bonds are shown as straight lines

• In H2O and HF, as in most molecules and polyatomic ions, nonmetal atoms except H are surrounded by eight electrons, an octet.

• In this sense, each atom has a noble gas structure

• Lewis structures are written following a stepwise procedure

A) Rules for writing Lewis Structures (single bonds)

1. Count valence electrons available. (use group number for nonmetals!)

2. Add electrons to take into account negative charge

• OCl- ion: 6 + 7 + 1 = 14 valence e-

• CH3OH molecule: 4 + 4(1) + 6 = 14 valence e-

• SO3-2 ion:

6 + 3(6) + 2 = 26 valence e-

Draw skeleton structure, using single bonds

• Note that carbon almost always forms four bonds. Central atom is written first in formula, terminal atoms are most often H, O, or a halogen

3.

Deduct two electrons for each single bond in the skeleton

OCl- ion: 14 – 2 = 12 valence e- leftCH3OH molecule: 14 – 10 = 4 valence e- leftSO3

-2 ion: 26 – 6 = 20 valence e- left

4.

• Distribute these electrons to give each atom a noble gas structure, if possible

Examples of central atom

• POCl3 P is central atom

• SO4-2

S is central atom

• SO3-2

S is central atom

• PO4-3

P is central atom

• SCl2 S is central atom

Draw Lewis Dot Structure

Ethane C2H6

Check answer on page 167

B. Too few electrons; form multiple bonds

• Structure of NO3- ion?

• Number of valence electrons = ?

• 5 + 18 + 1 = 24• Go to Elmo #4

Exceptions to the octet

• When we must exceed the octet, extra electrons go on central atom. (expanded octet)

• ClF3

• XeO3

• ICl4-

• BeCl2

• Consider XeF4, 36 valence electrons. Octet structure uses 32 electrons. Put extra e- around Xe

• ELMO 5• In a few molecules, there are less than

eight electrons around the central atom

Resonance

• Sometimes there is more than one valid structure for an molecule or ion.

• NO3-

• Transparency 6• True structure is a hybrid of those three

forms• Use double arrows to indicate it is the

“average” of the structures.• It doesn’t switch between them.

Note that…..

• Resonance forms are obtained by moving electrons, not atoms.

• Resonance can be expected when is possible to draw more that one structure that follows the octet rule

VSEPR

• Lewis structures tell us how the atoms are connected to each other.

• They don’t tell us anything about shape.• The shape of a molecule can greatly

affect its properties.• Valence Shell Electron Pair Repulsion

Theory allows us to predict geometry

VSEPR

• Molecules take a shape that puts electron pairs as far away from each other as possible.

• Have to draw the Lewis structure to determine electron pairs.

• bonding• nonbonding lone pair• Lone pair take more space.• Multiple bonds count as one pair.

Linear

Octahedral