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# VSEPR for 5 electron clouds (part 1)

In this video, we apply VSEPR theory to molecules and ions with five groups or “clouds” of electrons around the central atom. To minimize repulsions, five electron clouds will always adopt a trigonal bipyramidal electron geometry. Depending on how many of the clouds are lone pairs, the molecular geometry will be trigonal bipyramidal (no lone pairs), seesaw (one lone pair), T-shaped (two lone pairs), or linear (three lone pairs). This video focuses on the first two molecular geometries, trigonal bipyramidal and seesaw. Created by Jay.

## Want to join the conversation?

• Why is the bond angle between the equatorial lone pair and the axial fluorines still 90 degrees? Shouldn't the lone pair repel the axial fluorines so that the angles between axial fluorines and the equatorial lone pair be more than 90 degrees, therefore causing the angles between the equatorial fluorines and the axial fluorines to be less than 90 degrees?
• Sharp thinking! Those are the theoretical bond angles.
The lone pair repels all the bond pairs and does just as you predicted.
The F-S-F bond angle between the equatorial fluorines is reduced from 120 ° to 102 °.
The F-S-F bond angle between the axial fluorines is reduced from 180 ° to 173 °.
The F-S-F bond angle between the axial fluorines and the lone pair is increased from 90 ° to 93.5 °.
The F-S-F bond angle between the equatorial and axial fluorines is reduced from 90 ° to 86.5 °.
• Is the trigonal bipyramidal geometry just observed behaviour or is there like a mathematical proof, that this configuration minimises electron-electron repulsion?
• It's both. In these exercises we are trying to make our best predictions following some basic rules. Scientific experiments will test whether our predictions were correct or not. If they are (in most cases they are) than all right! If they aren't, we want to know why is that. There are many ways to find out spatial distribution of atoms in a molecule.
• why are the lone pairs of electrons around chlorine not counted as electron clouds?
• I think it is because he is talking from the point of view of the CENTRAL atom. It is the electron clouds immediately around phosphorus (the central atom) that gives PCl5's shape as a whole, according to VSEPR model. In other words, it will be useless to analyze PCl5 shape from the point of view of each individual chlorine atoms.
• At ...
Why is it okay for Phosphorus to have an expanded octet?
What is the relation between period 3 and the expanded octet of Phosphorus?
Thank you.!
• You will have to refer to a concept called VBT (Valence Bond Theory)
• In the last example why is the angle between the lone pair of electrons and the axial bonding electrons still 90 degrees? I thought the lone pair would repel the axial ones more... Thank you!
• In a geometry sense, we still consider the bond angles to be the same as the previous molecule; 90° between the axial and equatorial groups, and 120° between all the equatorial groups. In a chemistry sense these bond angles are different because, like you said, the lone pairs provide some repulsion which distorts the bond angles. Geometrically it is viewed as an ideal shape, but chemistry views it how it appears realistically taking into consideration nuanced electron interactions.

The actual distortion from the ideal bond angles differs for each molecule. In case you’re interested, for SF4 the axial-equatorial bond angles are 86.55°, and the equatorial bond angles are 101.6°.

Hope that helps.
• At Why is it said that the structure on the left is the right one? ALSO why is it that we have only two resonance structures for the molecule at ?
• These are isomers, not resonance structures.
The five orbitals have a trigonal bipyramidal geometry.
There are only two places where the lone pair can go — the axial ot the equatorial location.
The see-saw geometry is more stable because the lone pair in the axial location has less total repulsion from the other electrons.
• Why wouldn't the bond angles on SF4 be less than ideal if the lone pair repels the bonding pairs?
• Yes, that's true: the bond angles are "less than ideal" but, to be honest, that is the ideal arrangement for a molecule with a structure like that.
The structure of SF4 is actually a "seesaw" which is a variation of "trigonal bipyramidal" because there is this extra pair of electrons. The bond angles are also smaller
• In the molecular structure, does the Sulfur, the central atom of SF4, has four electrons that is bonded to F?

Is that why we can put two more valence electrons when there are left overs?

So, we can put up to four more valence electrons in a central atom?
• Sulfur is in the 3rd period, meaning it has a d-orbital to store up to 10 extra electrons. In general, the octet rule is less important for the 3rd period down because of these sorts of exceptions
(1 vote)
• How do you know when the octet rule applies? is it just when the formal charge isn't 0?
(1 vote)
• The octet rule applies for atoms in the second row of the periodic table (most notably: B, C, N, O, F, Ne). The first row (H and He) only has 1s orbitals available, so can only hold a maximum of 2 electrons. The beginning of the second row (Li, Be) often loses electrons to look more like the first row (i.e. 2 electrons). From the third row onwards, atoms can sometimes have more than an octet ("expanded octet"), since they now have d orbitals available, too.

Formal charges do not impact whether an atom follows the 'octet rule' or not.