Molecular Structures

General Chemistry

1. Molecular Structures

2. Learning Objectives At the end of the discussion, you should be able to: State and explain VSEPR Theory; Predict the structures of small molecules using valence shell electron pair repulsion (VSEPR) theory; Relate molecular geometry and physical/chemical properties; and, Determine the polarity of molecules

3. Molecular Geometry

4. Valence Shell Electron Pair Repulsion Theory (VSEPR Theory) Valence shell electrons of the central atom tend to take up positions that maximize their separation to attain stability. The valence shell electrons can either be a bonding pair or a lone pair . When these groups of electrons are situated close to each other, they tend to repel each other until repulsion is weakened with distance and the molecule becomes stable. The geometric structure acquired by any given molecule is actually a consequence of the electron repulsions of the different electron pairs that fill the valence shell of the central atom.

5. Linear Geometry Assumed by molecules with two groups surrounding the central atom. The group must be on opposite sides of the central atom with an ideal 180 o separation forming a linear shape.

6. Trigonal Planar For molecules with three surrounding groups. They are oriented in a plane separated by roughly 120 o .

7. Tetrahedral The four groups are located in the corners of a tetrahedron to make 109.5 o ideal angles in between any two groups.

8. Trigonal Bipyramidal The three groups located on the equatorial plane are separated from each other by 120 o ideal angles while the two groups on the axis are by a 180 o ideal angle.

9. Octahedral Four groups separated by 90o ideal angles lie along a plane (equatorial region) while the other two groups lie on both ends of an axis perpendicular to the plane (axial region).

10. Molecular Shapes Whereas molecular geometry is defined by total number of surrounding groups, the molecular shape is described in terms of surrounding atoms and not the surrounding lone pairs. It is possible for some molecules to have the same molecular geometries but have different shapes.

11. Molecular Geometries and Shapes

12. Sample Problem STEP 1. Write the Lewis Structure 2. Identify the Molecular Geometry by counting the total number of groups surrounding the central atom. Since NCl 3 has four surrounding groups, it is of a tetrahedral geometry. 3. Identify the number of bonded atoms X and number of lone pairs Y surrounding the central atom. X = 3 (three chlorine atoms) Y = 1 (one lone pair) 4. Identify the shape category using the formula AB x L y and predict the shape. Since the shape category is AB 3 L 1 , the shape is a trigonal pyramidal.

13. Seatwork: Draw the Lewis structure with indicating appropriate geometry for the following compounds: A. AsCl 3 B. SiF 5

14. Seatwork: Draw the Lewis structure with indicating appropriate geometry for the following compounds: A. AsCl 3 B. SiF 5