**What is VSEPR?**

VSEPR stands for Valence Shell Electron Pair Repulsion and it is a useful model for predicting the shapes and bond angles of molecules.

Electron pairs are negatively charged so they will repel each other if they get too close. The basic principle of VSEPR is that pairs of electrons will arrange themselves in space so as to cause the least amount of repulsion possible.

**Bonding pairs vs lone pairs**

Pairs of electrons can either be classified as bonding pairs (if they are located in a covalent bond shared between two atoms) or lone pairs (if they are left over after bonding has taken place). The important thing is that lone pairs repel more than bonding pairs. The order is as follows:

Lone pair-lone pair repulsion > lone pair-bonding pair repulsion > bonding pair-bonding pair repulsion

The reason for this difference is to do with the shapes of the orbitals these electrons occupy. Bonding pairs are localised in a bond and so are more constrained by where they can be. They take up less space than the lone pair, which is both more spread out and at a shorter distance to the nucleus. Letâ€™s look at some examples.

__Linear â€“ BeH2__

Be has 2 outer electrons so when it bonds to 2 Hs, it has no electrons left over.

The bonding pairs arrange themselves to be as far away from each other as possible, which in this case means 360/2 = 180 degrees apart.

*Bonding pairs: 2*

*Lone pairs: 0*

*Bond angle: 180*

__Trigonal planar â€“ BH3 __

Boron has 3 outer electrons, so when it bonds to 3 Hs, it has no electrons left over. The 3 bonding pairs of electrons arrange to be 360/3 = 120 degrees.

*Bonding pairs: 3*

*Lone pairs: 0*

*Bond angle: 120*

__Tetrahedral â€“ CH4__

Carbon has 4 outer electrons, so when it bonds to 4 Hs, there are no electrons left over. The bonds arrange to be as far apart as possible: 109.5 degrees away from each other.

*Bonding pairs: 4*

*Lone pairs: 0*

*Bond angle: 109.5*

__Pyramidal â€“ NH3__

Nitrogen has 5 outer electrons, so when it bonds to 3 Hs, there are 2 electrons (i.e. a lone pair) left over.

Now we can use a little trick to find the bond angle. The pyramidal structure is a bit like a tetrahedral structure (since it has 4 pairs of electrons in total) except it has 3 x bonding pairs and 1 x lone pair instead of 4 x bonding pairs. Since lone pairs repel more than bonding pairs, we can consider that this additional lone pair **reduces the bond angle by 2.5 degrees** compared to the tetrahedral structure. In this case:

109.5 â€“ 2.5 = 107 degrees

*Bonding pairs: 3*

*Lone pairs: 1*

*Bond angle: 107*

__V-shaped â€“ H2O__

Water is an example of a v-shaped molecule (also called non-linear or bent). Oxygen has 6 outer electrons, so when it bonds to 2 Hs, it has 4 electrons left over (i.e. 2 lone pairs). Like before, we can consider it to be a bit like tetrahedral (4 electrons pairs in total) but 2 of these are now lone pairs, so they repel more. The bond angle therefore **decreases by 2 x 2.5 degrees** compared to a tetrahedral structure:

109.5 â€“ (2 x 2.5) = 104.5 degrees

*Bonding pairs: 2*

*Lone pairs: 2*

*Bond angle: 104.5*

__Trigonal bipyramidal â€“ PF5__

This has two bond angles. The structure is a bit like a trigonal planar structure with two additional bonds cutting through the centre of the molecule at right angles to the other 3 bonds.

*Bonding pairs: 5*

*Lone pairs: 0*

*Bond angle: 90 and 120*

__Octahedral â€“ SF6__

When there are 6 bonds, all of them must be 90 degrees from each other.

*Bonding pairs: 6*

*Lone pairs: 0*

*Bond angle: 90*

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