II. Molecular Geometry Ch. 6 – Molecular shape

Molecules are three-dimensional objects that occupy a three- dimensional world; In general, only the smallest molecules can be said to have a fixed geometrical shape; the icosahedral C 60 “soccer ball” is a rare exception.

VSEPR Theory Valence Shell Electron Pair Repulsion Theory - focuses on the bonding and nonbonding electron pairs present in the outermost (“valence”) shell of an atom to which are connected two or more other atoms. Electron pairs orient themselves in order to minimize repulsive forces.

VSEPR Theory Types of e - Pairs –Bonding pairs - form bonds –Lone pairs - nonbonding e - Lone pairs repel more strongly than bonding pairs!!!

Draw the Lewis Diagram. Shape is determined by the # of bonding pairs and lone pairs. Know the 5 common shapes ! Determining Molecular Shape

1. Common Molecular Shapes 2 bond 0 lone LINEAR 180° AB 2

Linear molecules: AB 2 Ex: BeCl 2 and CO 2. -If you write out the electron dot formula for carbon dioxide, you will see that the C- O bonds are double bonds. Double bonds are treated like single bonds when shape is determined!!!!!!

CO 2 O C O 2 bond 0 lone LINEAR 180°

3 bond 0 lone TRIGONAL PLANAR AB 3 2. Common Molecular Shapes

Trigonal planar: AB 3 In the molecule BF 3, there are three regions of electron density extending out from the central boron atom. The repulsion between these will be at a minimum when the angle between any two is 120°. This requires that all four atoms be in the same plane; the resulting shape is called trigonal planar.

3. Common Molecular Shapes 2 bond 1 lone BENT AB 2 EAB 2 E 2

Shape with lone pairs: AB 2 E 2 2 bonding electrons and 2 lone pairs or AB 2 E, 2 bonding electrons and 1 lone pair Bent: The nonbonding electrons are also in orbitals that occupy space and repel the other orbitals.

OSeO SeO 2 SeO 2 IS V-SHAPED, OR BENT Se OO VSEPR treats double bonds like a single bond

Se OO OO RESONANCE!

2 bond 2 lone BENT 104.5° H2OH2O Common Molecular Shapes

4 bond 0 lone TETRAHEDRAL AB 4 4. Common Molecular Shapes

Tetrahedral: AB 4 Methane, CH 4, contains a carbon atom to which are connected four hydrogens. Consequently, the four equivalent bonds will point in four equivalent directions in three dimensions.

There is a better arrangement for four electron pairs: TETRAHEDRAL 4 electron pairs tetrahedral EPG C H H H H The shape of CH 4 is tetrahedral.

3 bond 1 lone TRIGONAL PYRAMIDAL AB 3 E 5. Common Molecular Shapes

PF 3 3 bond 1 lone TRIGONAL PYRAMIDAL F P F F

Polyatomic ions The charge will dictate how many less or extra electrons will be added to the lewis dot diagrams. NO 3 - the negative charge tells you that you need to add one more electron to the diagram (6) + 1 = 24 Polyatomic ions

Nitrate anion N OO O VSEPR treats multiple bonds as effective single electron pairs. Trigonal planar is the shape