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

This leads to even more possible molecular geometries (shapes). Just as it is possible for some molecules to have less than an octet, there is even more molecules that have more than an octet. This leads to even more possible molecular geometries (shapes). Consider PF5 for example.

It’s easy to see where three of the fluorine atoms can bond. ? It’s easy to see where three of the fluorine atoms can bond. PF5 F F P F But what about the 4th and 5th fluorine? Five groups of electrons form around the central atom. P F An electron from the s sublevel is promoted to the d. F F sp3d 3d 3s 3p The third principal energy level has an available 3d sublevel. What’s the shape of 5 groups of electrons arranged as far apart as possible? Hybridization of the s, the 3 p and a d orbital is called sp3d hybridization.

(The earth spins on its axis.) The other two are in the axial position, 90o from the equatorial groups. (The earth spins on its axis.) 90o The three groups arranged 120o from each other are in the equatorial position forming an equilateral triangle. 120o Equatorial positions Trigonal bipyramidal 5,5

The shape is called trigonal bipyramidal because it’s two three sided pyramids with a common base.

Sulfur tetrafluoride, SF4, is the next example. Two fluorines bond. But for the other two fluorines to bond, promotion of an s sublevel electron to the d sublevel takes place.

But one group is not bonding. F 5,4 Remember, the non-bonding group is larger since it’s not stretched out between two atoms. It would be in the equatorial position to minimize electron repulsion. (VSEPR) Removing an axial fluorine will result in a different shape than an equatorial one. Five groups of electrons means the groups are arranged in a trigonal bipyramidal geometry. The shape is called see-saw.

The shape is called see-saw because. . . . . . it rocks?

Cl F Let’s meet our next molecule, chlorine trifluoride, ClF3. The Lewis structure shows us that this is a 5,3 situation. The 5 groups are still arranged in a trigonal bipyramidal geometry. But only 3 of the groups are bonding.

The non-bonding pairs require the most room and that would be in the equatorial position, 120o apart. (instead of 90o) 180o 90o 5,3 T-shaped

Xe F 5,2 Triatomic Linear Xenon difluoride would be 5,2 Another trigonal bipyramidal geometry but with only two bonding groups. 180o 5,2 Triatomic Linear

Sulfur hexafluoride is an example of a molecule that has 6 valence groups. But for 4 more to bond, the s and the p electrons are promoted to the d sublevel. The one s, three p and two d orbitals blend to form new orbitals called sp3d2. F Two fluorine atoms bond here. F sp3d2 5s 5p 5d

According to the VSEPR theory the six orbitals minimize electron repulsion by arranging themselves as far apart as possible. One on top, one on bottom, one on the left, one on the right, So what is that geometry? one in the front, one in the back,

6,6 = Octahedral It’s called octahedral because…. and four sides on the top. it has four sides on the bottom…. All angles are 90o (a four sided pyramid) 6,6 = Octahedral

BrF5 is our next example. Bromine has 7 valence electrons. Promotion of an s electron and a p electron to the d sublevel, allows bonding of 4 more fluorine atoms. F F F Br F F 6 groups of electrons, 5 are bonding.

The 6 groups are in an octahedral geometry with one non-bonding pair. We would put the non-bonding group in the position that allows it the most room. But in this case, all positions are equally far apart. And the shape would be the same no matter which group is the non-bonding group.

6,5 Square pyramidal

Xenon tetrafluoride, XeF4 6,4 Square planar

5,5 Trigonal pyramidal sp3d 5,4 Seesaw sp3d 5,3 T-shaped sp3d 5,2 Triatomic Linear sp3d

6,6 Octahedral sp3d2 6,5 Square pyramidal sp3d2 6,4 Square planar sp3d2