Mark S. Cracolice
Edward I. Peters
www.cengage.com/chemistry/cracolice
Chapter 13
Structure and Shape
Mark S. Cracolice • The University of Montana
Drawing Lewis Diagrams
1 Count the total number of valence electrons. If the
species is an ion, the number of valence electrons
must be adjusted for the charge on the ion.
Determine the number of electron pairs.
2 Place the least electronegative atom in the center of
the molecule.
3
Draw a tentative diagram. Place one electron pair
between each pair of bonded atoms.
Drawing Lewis Diagrams
4 Determine the number of lone pairs by subtracting
the number of single bonds from the total number of
electron pairs available.
5 Starting at the outer atoms, distribute the available
lone pair electrons to complete the octet around each
atom except hydrogen which requires a duet.
6 If there are not enough electrons to complete all
octets, move one or more lone pairs from an outer
atom to form a double or triple bond with the central
atom until all atoms have an octet.
Drawing Lewis Diagrams
Example: Draw the Lewis diagram for carbon
tetrafluoride.
First, count valence electrons.
CF4: 4 (C) + 4 × 7 (F) = 32 valence electrons
There are 16 electron pairs
Second, determine the central atom by comparing
electronagativity
EN of C < EN of F, so C is central
Drawing Lewis Diagrams
Draw a tentative diagram with all single bonds and lone
pairs..
Drawing Lewis Diagrams
Check
Carbon has an octet (4 bonding pairs = 8 electrons)
Each fluorine has an octet (one bonding pair and three
lone pairs)
Valence Shell Electron-Pair Repulsion
Theory, VSEPR.
Electron pairs repel one another.
Therefore they distribute themselves in positions
around a central atom that are as far away from each
other as possible.
These are the locations of lowest potential energy.
Electron-Pair Geometry
1. Count the number of lone electron pairs and
the number of bonding electron pairs around
the central atom. The total number of electron
pairs is called the steric number, SN.
2. Distribute the electrons pairs in positions
that are far away from each other as possible.
These are the locations of lowest energy.
Electron-Pair Geometry
Electron-Pair Geometry
Arrangement of regions of electron density
(electron pairs) around a central atom in a molecule.
Electron-Pair Angle
The geometric angle formed by any two
electron pairs and the central atom.
Electron-Pair Geometry
a. The central atom has two electron pairs the electron pair
geometry is linear. The electron pair angle is 1800
b. The central atoms is surrounded by three electron pairs. The
geometry is trigonal planar. The electron pair angle is 1200
c. The central atom is surrounded by four electron pairs, The
geometry is tetrahedral, and the electron pair angle is 109.50
Molecular Geometry
Molecular Geometry
Arrangement of atoms around a central atom in
a molecule.
Molecular geometry follows from electron-pair
geometry.
Molecular Geometry: BeF2
Regions of electron density: 2
Electron-pair geometry: Linear
Regions bonded: 2
Molecular geometry: Linear
Molecular Geometry: BF3
Regions of electron density: 3
Electron-pair geometry: Trigonal planar
Regions bonded: 3
Molecular geometry: Trigonal planar
Molecular Geometry: SO2
Regions of electron density: 3
Electron-pair geometry: Trigonal planar
Regions bonded: 2
Molecular geometry: Angular
Molecular Geometry: CH4
Regions of electron density: 4
Electron-pair geometry: Tetrahedral
Regions bonded: 4
Molecular geometry: Tetrahedral
Molecular Geometry: NH3
Regions of electron density: 4
Electron-pair geometry: Tetrahedral
Regions bonded: 3
Molecular geometry: Trigonal pyramidal
Molecular Geometry: H2O
Regions of electron density: 4
Electron-pair geometry: Tetrahedral
Regions bonded: 2
Molecular geometry: Bent
Molecular Geometry
Molecular Geometry
Conventions Used for Drawing Wedge-and-Dash Diagrams
Polarity of Molecules
Polar Molecule
A molecule in which there is an
asymmetrical distribution of charge.
Nonpolar molecule
A molecule in which there is a symmetrical distribution
of charge.
Polarity of Molecules
In an electric field, polar molecules are oriented with the
positive end toward the direction of the field.
Polarity of Molecules
Polarity of Molecules
Homework
• Prepare Exercise B