H C Valence Shell Electron Pair Repulsion Theory 2p 2s 1s Ken Rogers Miami Killian Part 1

We’ll start with methane, CH4. VSEPR theory is used to predict the shape of a molecule. In order to do this, however you need to be able to draw an electron dot structure for the molecule. After that, just count the electron groups around the central atom and count those groups that are bonding groups. We’ll start with methane, CH4.

C H C The electron configuration of carbon is 1s2 2s2 2p2. The orbital diagram shows the electrons in the 2s sublevel are paired. 1s 2s 2p In order for carbon to bond with hydrogen to form CH4, a 2s orbital electron would need to move to a 2p sublevel. H C

This process of the 2s electron moving to a 2p sublevel is called promotion. The orbitals and their shapes change. The new orbitals have a new shape and the blending of the s orbital with the three 2p orbitals is called hybridization.

H C 4 valence orbitals repel themselves as far apart as possible. And 4 orbitals spread out as far as possible in 3 dimensional space is not : the real shape is: All bond angles are 109.5o

Z X Y s sp3 px,py,pz The blending of an s orbital with three p orbitals results in the hybridization type known as sp3 hybridization.

Tetrahedral 4, 4 = tetrahedral Any electron dot structure that has 4 orbitals and all 4 are bonding is tetrahedral shaped. 4, 4 = tetrahedral

Now, the electron dot structure for ammonia, NH3 begins with N, Promotion is not necessary to bond the hydrogens. 4,3 = ? The number of orbitals is 4, but only 3 of them are bonding.

The bonding does not need promotion. The hybridization is still sp3. All molecules with 4 orbitals have sp3 hybridization. The non bonding orbital is not stretched out between the atoms so it is larger than the other orbitals. The larger non-bonding orbital repels the other 3 orbitals closer together. Bond angles are 107o

The 4 atoms form a 3 sided pyramid 4, 3 = trigonal pyramidal The 4 atoms form a 3 sided pyramid

Water is also sp3 hybridization. 4 orbitals, only 2 bonding. H 4,2 4, 2 = bent Bond angles are 104.5o

Cl H 4, 1 = linear 4 orbitals, only 1 bonding. No bond angles (An angle requires 3 points.)

To summarize so far: Copy this chart. #of orbitals, bond type of Example # bonding shape angle hybrid CH4 4,4 tetrahedral 109.5o sp3 NH3 4,3 trigonal pyramidal 107o sp3 H2O 4,2 bent 104.5o sp3 HCl 4,1 linear none sp3 More to follow

F B F F 3,3 = ? The electron dot structure for BF3 is BF3 is a molecule that doesn’t achieve an octet. F B F F 3,3 = ? The number of orbitals around the central atom is 3, all 3 of are bonding.

This is a new hybridization type. Boron has two of its electrons paired in the 2s sublevel. Promotion of a 2s e- to a 2p sublevel occurs. B 2sp2 2s 2p 1s This is a new hybridization type. sp2 hybridization

F B F And produces three orbitals arranged as far apart as possible. F 3,3 This is why the theory is called Valence Shell Electron Repulsion. How do you arrange 3 orbitals as far apart as possible?

Bond angles are 120o 120o 120o 3, 3 = trigonal planar

S O 3,3 = trigonal planar The electron dot structure for SO3 is SO3 achieves an octet by forming a double bond. O S 3,3 = trigonal planar The number of orbitals around the central atom is 3 because the one double bond is considered one bond. All three are bonding.

S O 3,3 = trigonal planar This is the ball and stick model of SO3.

The electron dot structure for SO2 is 3,2 = ? The number of orbitals around the central atom is 3 but only two are bonding.

The nonbonding pair repels the bonding pairs closer together. Bond angle about 117o 3,2 = bent

Molecular oxygen would have the following shape. No bond angle 3,1 = linear

It’s not necessary to go through all the promotion and hybridization steps to figure out the shape of a molecule. The shape can be determined from its electron dot structure. 1. Draw the electron dot structure. 2. Count the number of electron groups around the central atom. Then count the groups that are bonding. Double bonds and single bonds are considered one group.

C O C O 2,2 triatomic linear Consider carbon dioxide, CO2. Around the central atom, are 2 electron groups. 2 are bonding. Two electron groups repelling themselves as far apart as possible would result in a linear arrangement. The shape is triatomic linear.

180o

The electron dot structure for N2 is: 2,1 = linear sp hybridization

Finish your chart. To summarize #of orbitals, bond type of Example # bonding shape angle hybrid BF3 SO3 3,3 trigonal planar 120o sp2 SO2 3,2 bent 117o sp2 O2 3,1 linear none sp2 CO2 2,2 triatomic linear 180 sp N2 2,1 linear none sp