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Mark S. CracoliceEdward I. Peters

Mark S. Cracolice • The University of Montana

Chapter 13Structure and Shape

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 electronsThere 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 GeometryArrangement of regions of electron density

(electron pairs) around a central atom in a molecule.

Electron-Pair AngleThe 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 GeometryArrangement of atoms around a central atom in

a molecule.

Molecular geometry follows from electron-pair geometry.

Molecular Geometry: BeF2

Regions of electron density: 2Electron-pair geometry: Linear

Regions bonded: 2Molecular geometry: Linear

Molecular Geometry: BF3

Regions of electron density: 3Electron-pair geometry: Trigonal planar

Regions bonded: 3Molecular geometry: Trigonal planar

Molecular Geometry: SO2

Regions of electron density: 3Electron-pair geometry: Trigonal planar

Regions bonded: 2Molecular geometry: Angular

Molecular Geometry: CH4

Regions of electron density: 4Electron-pair geometry: Tetrahedral

Regions bonded: 4Molecular geometry: Tetrahedral

Molecular Geometry: NH3

Regions of electron density: 4Electron-pair geometry: Tetrahedral

Regions bonded: 3Molecular geometry: Trigonal pyramidal

Molecular Geometry: H2O

Regions of electron density: 4Electron-pair geometry: Tetrahedral

Regions bonded: 2Molecular geometry: Bent

Molecular Geometry

Molecular GeometryConventions Used for Drawing Wedge-and-Dash Diagrams

Polarity of Molecules

Polar MoleculeA molecule in which there is an

asymmetrical distribution of charge.

Nonpolar moleculeA 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