Lewis Structures & VSEPR
Meyer 2013
Lewis Structures
• Lewis Structure
– Picture of a molecule
– Uses lines to indicate chemical bonds
– Uses dots to indicate electrons
– Shows 2 dimensional image of molecule
Basic Rules
• All atoms want 8 electrons (hydrogen is the
only exception it wants 2)
• Hydrogen & halogens are never central atoms
• Carbon tends to be a central atom
The Official rules for Lewis structures
1. Calculate the total number of valence e- add all the # valence e- for each atom
– If you are writing a Lewis structure for an ion with
a + or – charge, be sure to add or subtract
electrons as the charge indicates.
The Official rules for Lewis structures
2.
Write the skeleton structure for the
molecule or ion, connect every bonded
pair of atoms with a dash. (Hint: the least
electronegative atom is the central atom)
The Official rules for Lewis structures
3. Distribute electrons to the atoms
surrounding the central atom (or atoms) to
satisfy the octet rule for these surrounding
atoms.
The Official rules for Lewis structures
4.
If fewer than eight electrons are present
around the central atom, a double or triple
bond may be necessary.
• To obtain a double or triple bond, move one or two
electron pairs from a surrounding atom to bond with
the central atom.
• Sulfur, carbon, oxygen, and nitrogen all typically form
double bonds. Carbon and nitrogen also form triple
bonds.
Practice Problems
Draw a Lewis structure for each:
•
•
•
•
•
Carbon Tetrachloride
Hydrogen sulfide
Sulfate ion
Carbon dioxide
Carbonate ion
Practice Problems
Draw a Lewis structure for each:
•
•
•
•
•
Carbon Tetrachloride CCl4
Hydrogen sulfide H2S
Sulfate ion SO4-2
Carbon dioxide CO2
Carbonate ion CO3-2
VSEPR Theory
• Valence-Shell Electron-Pair Repulsion (VSEPR) is a simple
method for determining geometry.
• Basis: pairs of valence electrons in bonded atoms repel
one another.
• These mutual repulsions push electron pairs as far from
one another as possible.
When the electrons
B
B
A
B
B
B
A
B
bond, the elements
are as far apart as
they can get. What
is the B-A-B angle?
A Balloon Analogy
• Electron groups repel
one another in the
same way that
balloons push one
another apart.
• When four balloons,
tied at the middle,
push themselves
apart as much as
possible, they make a
tetrahedral shape.