VSPER

VSPER Stands for Valance Shell Pair Electron Repulsion The whole idea is that electrons repel each other Electrons are attached to the central atom in a molecule but then move away from each other

Types of electron Domains – areas of electron concentration There are three basic types of domains Bonded Pairs Non bonding Pairs Single electrons The repulsion between electrons helps to determine the shape of molecules

EX: H2O H –O –H 2 pair of e- are involved in the bonding, these are shared pairs. 2 pair are not bonded, these are called unshared pairs. What type of shape does this molecule assume? Can it be predicted?

VSEPR Theory Valence shell, electron-pair repulsion (VSEPR) Theory states that repulsion between the sets of valence-level electrons surrounding an atom causes these sets to be oriented as far apart as possible. Using VSEPR, we can predict the geometries of molecules.

VSEPR Theory to Predict Molecular Geometries Step 1: Write the Lewis Dot Structure for the molecule. Step 2: Count the domains. Step 3: Count the number of unbounded pairs. Step 4: Now refer to Table for VSEPR and Molecular Geometries

Lewis Dot Diagram of CCl4 EX: CCl4 1. Lewis Dot: C has 4 valence and Cl has 7 x 4 for a total of 32 e-. 2. Place C in center. 3. Place Cl around C. 4. Draw bonds connecting C and Cl. 5. Subtract bonding e-s from total e- (32 – 8) = 24 e-

6. Now put electrons around the most electronegative (usually outermost) atoms (Cl) first, try to satisfy octet rule. 7. There are no remaining e-.

The Lewis Dot for CCl4 Cl Cl C Cl

Predicting Molecular Geometry of CCl4 Refer to structure. How many electron pairs are present – how many domains? Four. 2. How many atoms are bonded to the central atom? 3. According to the table, the shape for CCl4 is tetrahedral.

Next Example: PCl3 1. Lewis Dot Diagram: Cl Cl P Cl 2. P is the central atom. There are 4 sets of electrons, but only 3 are bonding sets. 3. Refer to your table. What is the shape???

Molecular Geometry of H2O Remember, H2O has a Lewis Structure of H—O—H The O is the central atom. There are 4 domains, but two are unshared pairs. What is the shape?

Structure of BCl3 Cl Cl B Cl Note: Boron is an exception to the octet rule –it is stable in some compounds with only 6 electrons.

Structure of BCl3 Cl Cl B Cl With 3 domains and no lone pairs the shape is trigonal planar