VSEPR THEORY (Valence Shell Electron Pair Repulsion Theory) Take notes on the slides Mrs Jacobus Adapted from Mr. M. McIsaac Carleton North High School, Bristol, NB

2 What Is The VSEPR Theory? VSEPR Theory is used to predict the shapes of molecules. VSEPR Theory is used to predict the shapes of molecules. Think of bonded pairs or BP (shared) or lone pairs or LP (nonbonded, unshared) Think of bonded pairs or BP (shared) or lone pairs or LP (nonbonded, unshared) e - ’s are negatively charged clouds that repel each other. e - ’s are negatively charged clouds that repel each other. To achieve the most stable condition: To achieve the most stable condition: clouds must be as far apart as possible in 3-D, thereby decreasing repulsion. clouds must be as far apart as possible in 3-D, thereby decreasing repulsion. The amount of repulsion can be ordered: The amount of repulsion can be ordered: LP-LP > LP-BP > BP-BP In order to determine the shape, the Lewis dot structures must be drawn first. In order to determine the shape, the Lewis dot structures must be drawn first.

3 2 Bond Pairs/Electron Groups Molecules that only have 2 bonding pairs(BP) on the central atom will have a LINEAR SHAPE with a bond angle of 180° Molecules that only have 2 bonding pairs(BP) on the central atom will have a LINEAR SHAPE with a bond angle of 180° e.g. BeF 2, CO 2, CS 2 e.g. BeF 2, CO 2, CS 2 General Formula: AX 2 General Formula: AX 2 Central atom A from group 2; 2 BP 0 LP Central atom A from group 2; 2 BP 0 LP

4 3 Bond Pairs/Electron Groups Molecules that have 3 bonding pairs on the central atom will have a TRIGONAL PLANAR SHAPE with bond angles of 120°. Molecules that have 3 bonding pairs on the central atom will have a TRIGONAL PLANAR SHAPE with bond angles of 120°. e.g. BF 3, BH 3 e.g. BF 3, BH 3 General Formula: AX 3 General Formula: AX 3 Central atom A from group 13; 3 BP 0 LP Central atom A from group 13; 3 BP 0 LP

5 4 Bonding Pairs/Electron Groups If the central atom is placed at the center of a sphere, than each of the four pairs of electrons will occupy a position to be as far apart as possible. If the central atom is placed at the center of a sphere, than each of the four pairs of electrons will occupy a position to be as far apart as possible. results in the electron pairs being at the corners of a regular tetrahedron results in the electron pairs being at the corners of a regular tetrahedron these molecules are said to have a TETRAHEDRAL SHAPE. these molecules are said to have a TETRAHEDRAL SHAPE. The angle between each bond will be 109.5° The angle between each bond will be 109.5° e.g. CCl 4, CH 4, SiH 4 e.g. CCl 4, CH 4, SiH 4 General Formula: AX 4 General Formula: AX 4 Central atom A from group 14; 4 BP 0 LP Central atom A from group 14; 4 BP 0 LP

6 Example CCl 4

7 3 Bonding Pairs(BP) & 1 Non-bonding (LP) Pair Four pairs of electrons will always arrange themselves tetrahedrally around the central atom. Four pairs of electrons will always arrange themselves tetrahedrally around the central atom. The shape of the molecule is determined by the arrangement of the atoms not the electrons. The shape of the molecule is determined by the arrangement of the atoms not the electrons. As a result such molecules will have a TRIGONAL PYRAMIDAL shape. As a result such molecules will have a TRIGONAL PYRAMIDAL shape. Due to the repulsion, a non-bonding electron pair (LP) requires more space than a bonding pair (BP), the angles in these molecules are 107° not 109.5° as in the tetrahedral molecules. Due to the repulsion, a non-bonding electron pair (LP) requires more space than a bonding pair (BP), the angles in these molecules are 107° not 109.5° as in the tetrahedral molecules. e.g. NH 3, PCl 3 e.g. NH 3, PCl 3 General Formula: AX 3 E General Formula: AX 3 E Central atom A from group 15; 3 BP 1 LP Central atom A from group 15; 3 BP 1 LP

8 Example NH 3

9 2 Bonding Pairs (BP)& 2 Non-bonding Pairs (LP) The four pairs of electrons will be arranged tetrahedrally but since only 2 pairs are bonding electrons, the surrounding atoms are at 2 corners of the tetrahedron. The four pairs of electrons will be arranged tetrahedrally but since only 2 pairs are bonding electrons, the surrounding atoms are at 2 corners of the tetrahedron. As a result these molecules will have a V-SHAPE or BENT. As a result these molecules will have a V-SHAPE or BENT. The repulsion between the non-bonding pairs (LP) will result in a bond angle of 104.5°. The repulsion between the non-bonding pairs (LP) will result in a bond angle of 104.5°. For each pair of non-bonding electrons, the bond angle decreases by 2.5° For each pair of non-bonding electrons, the bond angle decreases by 2.5° e.g. H 2 O, H 2 S, OCl 2 e.g. H 2 O, H 2 S, OCl 2 General Formula: AX 2 E 2 General Formula: AX 2 E 2 Central atom A from group 16; 2 BP 2 LP Central atom A from group 16; 2 BP 2 LP

10 Example H 2 O

Go to the following website: Complete the attached worksheet and be sure to click on each of the animated shapes Complete the attached worksheet and be sure to click on each of the animated shapes..\..\Honors Chemistry 2007 and on\CH 7- Bonding and Molecular Geometry\Molecular Geometry Activity.doc..\..\Honors Chemistry 2007 and on\CH 7- Bonding and Molecular Geometry\Molecular Geometry Activity.doc..\..\Honors Chemistry 2007 and on\CH 7- Bonding and Molecular Geometry\Molecular Geometry Activity.doc..\..\Honors Chemistry 2007 and on\CH 7- Bonding and Molecular Geometry\Molecular Geometry Activity.doc 11