¡ Properties of molecules not only depend on the bonding of atoms but also on molecular geometry

§ Molecular geometry: the three-dimensional arrangement of a molecule’s atoms in space

¡ “Valence-Shell Electron-Pair Repulsion”§ Refers to the repulsion between pairs of valence electrons of

the atoms in a molecule

¡ VSEPR theory: repulsion between the sets of valence-level electrons surrounding an atom causes these sets to be oriented as far apart as possible

¡ VSEPR theory is used to predict the shapes of molecules based on the fact that electron pairs strongly repel each other and tend to be oriented as far apart as possible

VSEPR and Unshared Electron Pairs:¡ VSEPR theory suggests that the lone pair occupies space

around an atom just as bonding pairs do¡ Actual shape of a molecule is determined by the positions of the

atoms only

§ Ex: NH3

§ In VSEPR theory, double and triple bonds are treated in the same way as single bonds

¡ Linear§ Atoms bonded to central atom: 2§ Lone pairs of electrons: 0 § Bond angles: 180°§ Generic formula: AX2

¡ Trigonal Planar§ Atoms bonded to central atom: 3§ Lone pairs of electrons: 0 § Bond angles: 120°§ Generic formula: AX3

¡ Bent§ Atoms bonded to central atom: 2§ Lone pairs of electrons: 1§ Bond angles: <120°§ Generic formula: AX2E

¡ Tetrahedral§ Atoms bonded to central atom: 4§ Lone pairs of electrons: 0 § Bond angles: 109.5°§ Generic formula: AX4

¡ Trigonal-Pyramidal§ Atoms bonded to central atom: 3§ Lone pairs of electrons: 1 § Bond angles: 107°§ Generic formula: AX3E

¡ Bent§ Atoms bonded to central atom: 2§ Lone pairs of electrons: 2 § Bond angles: 104.5°§ AX2E2

¡ Trigonal-Bipyramidal§ Atoms bonded to central atom: 5§ Lone pairs of electrons: 0 § Bond angles: 120°, 90°§ AX5

¡ Octahedral§ Atoms bonded to central atom: 6§ Lone pairs of electrons: 0 § Bond angles: 90°§ AX6

Use VSEPR theory to predict the molecular geometry of AlCl3.1. Draw the Lewis structure for the molecule.

2. Look at the number of bonds and lone electron pairs made in the molecule.

§ How many bonds are attached to the central atom?§ Three

§ How many lone electron pairs are there?§ None

3. Look at the type of molecule it is and refer to the chart to see what type of molecular shape it has.§ AlCl3 = AX3 = Trigonal-planar

¡ Use VSEPR theory to predict the molecular geometry of the following molecules:§ A) HI▪ Linear

§ B) CBr4

▪ Tetrahedral§ C) AlBr3

▪ Trigonal-planar§ D) CH2Cl2▪ Tetrahedral

§ E) CO2 [double bond]▪ Linear

¡ Factors that affect the geometry of a molecule:- The number of bonds formed by each atom in the molecule- The number of lone pairs of electrons on the atoms- The sizes of the various types of atoms- The hybridization of some of the atoms’ orbitals

§ 1. SeH2

▪ A) # bonded atoms▪ B) # lone pairs▪ C) Shape

§ 2. CF4

▪ A) # bonded atoms▪ B) # lone pairs▪ C) Shape

§ 3. PBr3

▪ A) # bonded atoms▪ B) # lone pairs▪ C) Shape

§ 4. BCl3

▪ A) # bonded atoms▪ B) # lone pairs▪ C) Shape

§ 5. NI3▪ A) # bonded atoms▪ B) # lone pairs▪ C) Shape

§ 6. CH3Br▪ A) # bonded atoms▪ B) # lone pairs▪ C) Shape

§ 7. HCN▪ A) # bonded atoms▪ B) # lone pairs▪ C) Shape

§ 8. Na2O▪ A) # bonded atoms▪ B) # lone pairs▪ C) Shape

Directions: Determine the Lewis structure of the following molecules. Indicate the number of atoms and number of lone pairs bonded to the central atom, and use the VSEPR theory to predict the molecular geometry shape of the respective molecules.