Lewis Dot Structures & VSEPR Theory
Chemistry
Ms. Piela
Lewis Dot
 Shows the number of valence electrons for
a given atom
 Rules
 Each atom is represented by its symbol
 No more than 2 electrons on each side of the
element’s symbol
 Only the valence electrons appear in a Lewis Dot
Examples
 H
 F
Practice
 C
 B
 Br
 Ne
 O
Lewis Dot Structures
 General Strategy
 Count up the total number of valence electrons
 Draw at least one bond to the central atom (each
bond counts as 2 electrons)
 Complete the octets of atoms attached to the
central atom (remember: electrons only work in
pairs)
 If there are not enough electrons to satisfy octets consider
using double/triple bonds
Tips for Lewis Dot Greatness
 Double/Triple bonds mostly form between C, N, O, and
S
 H, Be, and B are special exception to the octet rule.
They do not follow it!
 The central atom is the atom that forms the most
bonds, or is usually written first
 Elements beyond the 2nd period may have more than 8
valence electrons (not really seen here)
Examples
 Cl2
 NH3
 CH4
Practice
 CF4
 H2O
 HF
 BF3
Complex Lewis Dot Structures
 More than one single bond is possible
 Double bond – two bonds between atoms
(4 electrons total)
 Triple bond – three bonds between atoms
(6 electrons total)
 Bond length decreases as you increase the number
of bonds
 Bond strength increases as you increase the number
of bonds
Examples
 CO2
 CH2O
Practice
 SO3
 N2
 HCN
Determining Molecular Shapes
 VSEPR (Valence Shell Electron Pair
Repulsion) Theory –
A model that accounts for geometrical
arrangements of shared and unshared
electron pairs around a central atom
 Shapes are determined by repulsive forces
between shared and unshared electrons
VSEPR Theory
 To determine molecular geometry
(shape), count the electron pairs
surrounding the central atom
 Electron pairs = (number of lone pairs on
central atom) + (number of atoms
bonded to central atom)
Examples
 BH3
 NH3
 CH4
Examples
 H2O
 SO2
Practice
 SiF4
 BF3
 HCN
 SO42-