Basic Terminology

 Molecule
 Group of atoms held together by covalent bonds
 Bond Length
 Distance between 2 bonded atoms
 Bond Energy
 Energy required/needed to break a chemical bond
 Bond Angles
 Determined based on bonding and nonbonding
regions of the central atom in a molecule
Molecular Shape

 3-D structure of a molecule
 Predicted from the Lewis dot diagram of a chemical
compound
 Influenced by the number of valence electron pairs AND
number of electrons existing as bonded or lone pairs.
 Exact shape experimentally determined
 Vital characteristic of a chemical compound
Valence-Shell Electron-Pair
Repulsion(“VSEPR”) Method
Also called “Electron-Domain

Model”
 Valence electrons repel each other in chemical bond
 Valence electron pairs are arranged so the distance between
them is MAXIMIZED !
 Valence electron pair number (bonded vs. lone)
 Influence atom arrangement around central atom
 Indirect influence on shape
 Focus on central atom
 Shared electrons
 Unshared electrons
VSEPR or ElectronDomain Model (cont.)

 Based on electron pair domains
 Bonding Domains
 2 atoms share domain
 Electrons are shared within domain
 Nonbonding Domains
 Domain only exists within valence shell of an individual
atom
 Electrons are not shared
***Molecular structures attempt to keep bonding domains as
far apart as possible***
What about unshared
electrons on the central

atom?
 Lone/unshared electrons
 Occupy space around central atom
 Not involved in chemical bonding between atoms
 These pairs are NOT taken into account for a chemical
molecule’s shape !
 Require MORE space than shared electrons, indirect
contribution to shape
1) Electron Groups

 Group of valence electrons “hanging out” around central
atom, orientation around central atom
 Describes how electron groups are distributed around central
atom
 Composition: unpaired, lone pair, one/two/three bound
pairs
 Orientations based on electron groups





Linear—2
Trigonal planar—3
Tetrahedral—4
Trigonal bipyramidal—5
Octahedral--6
2) Molecular Geometry

 Describes the atom arrangement around a central
atom based on chemical bonds, dealing with
BONDED atoms
 Orientation of BOUND atoms around a central atom
 Based on electron-group geometry
VSEPR Notation

 Central Atom = A
 Terminal atoms (bonded atoms) = X
 Lone pair electrons = E
 Example: Water
How do we draw
molecular shapes?

1) Draw Lewis structure
2) Identify number of electron groups—bound or lone
pair?
1) Determine electron group geometry
2) Figure out molecular geometry
 How many bonded atoms are around the central atom
 Is this geometry the same as electron group geometry?

Table 10.1 (pp. 390-391)
Handout
No Lone Pair Electrons

 Molecular geometry = Electron group geometry !!!
1) Linear (AX2)
 Bond angle = 180°
 2 electron groups
 Examples: MgCl2, CO2
No Lone Pair Electrons

2) Trigonal Planar (AX3)
 Bond angle = 120°
 3 electron groups
 Ex. BF3
3) Tetrahedral (AX4)
 Bond angle = 109. 5°
 4 electron groups
 Ex. CH4, CCl4
No Lone Pair Electrons

4) Trigonal bipyramidal (AX5)
 5 electron groups
 Only seen with expanded valence shell, period 3+
5) Octahedral (AX6)
 6 electron groups
 Only seen with expanded valence shell, period 3+
**Refer to Table 10.1 pp. 390-391
Lone Pair Electrons
Present

1) Trigonal planar with 1 lone pair (AX2E)




3 electron groups, one lone pair
How many bound atoms?
Bond angle= 120°
Known as “angular”
 Ex. SO2
Lone Pair Electrons
Present

2) Tetrahedral with 1 lone pair (AX3E)




4 electron groups (3 pairs, 1 lone pair)
How many bound atoms?
Known as “trigonal pyramidal”
Bond angle = 109.5°, 107°(actual)
 Ex. NH3
Lone Pair Electrons
Present

2) Tetrahedral with 2 lone pair (AX2E2)




4 electron groups (2 pairs, 2 lone pair)
How many bound atoms?
Known as “angular/bent”
Bond angle ≈ 104/105°
 Ex. H2O
Complex Molecules

 More than one central atoms is present
 Figure out molecular geometry for each central atom
 Combine BOTH geometries together to get the
overall shape
 Ex. HNO3
VSEPR Method “Take
Home” Points

 1) Repulsion between electron groups increases as
the groups’ distance decreases.
 2) Lone pair electrons have more repulsion than
bonded electrons.
 Charge can spread
 Need more space in shape
Homework

 Read pp. 387-398
 Problems
 p. 382 #59-60
 p.427 #21, 23, 25, 27, 29, 30