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
What Is The VSEPR Theory? • VSEPR Theory is used to predict the shapes of molecules. • Think of bonded pairs or BP (shared) or lone pairs or LP (nonbonded, unshared) • e-’s are negatively charged clouds that repel each other. • To achieve the most stable condition: • clouds must be as far apart as possible in 3-D, thereby decreasing repulsion. • 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.
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° • e.g. BeF2, CO2, CS2 • General Formula: AX2 • Central atom A from group 2; 2 BP 0 LP
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°. • e.g. BF3, BH3 • General Formula: AX3 • Central atom A from group 13; 3 BP 0 LP
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. • results in the electron pairs being at the corners of a regular tetrahedron • these molecules are said to have a TETRAHEDRAL SHAPE. • The angle between each bond will be 109.5° • e.g. CCl4, CH4, SiH4 • General Formula: AX4 • Central atom A from group 14; 4 BP 0 LP
3 Bonding Pairs(BP) & 1 Non-bonding (LP) Pair • 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. • 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. • e.g. NH3, PCl3 • General Formula: AX3E • Central atom A from group 15; 3 BP 1 LP
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. • 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°. • For each pair of non-bonding electrons, the bond angle decreases by 2.5° • e.g. H2O, H2S, OCl2 • General Formula: AX2E2 • Central atom A from group 16; 2 BP 2 LP
Go to the following website: http://www.chemmybear.com/shapes.html • 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