1.3.6

# Shapes of Molecules

Test yourself

## Valence Shell Electron Pair Repulsion Theory (VSEPR)

VSEPR helps to explain molecule shapes by considering electron-pair replusion.

### Charge clouds

-Electrons occupy orbitals. But what is an orbital?

• An orbital is just a region of space where you are likely to find an electron.
• The usual definition is that an orbital is a region of space where there is a 95% chance the electron is located.
• This means you can view electrons not as particles, but as charge clouds - regions of space where the electrons move around.

### Repulsions

• Electrons are negatively charged.
• This means that they repel each other.
• Electrons will try to stay as far apart from each other as possible.
• This determines the geometry of a molecule. The electrons in the bonds repel and try to stay as far from each other as possible.

### Lone pairs vs bonding pairs

• An important detail is differentiating between the repulsion of lone pairs and bonding pairs of electrons.
• Lone pairs are held closer to the nucleus of an atom. This means they repel each other more as they are physically closer.
• The trend in repulsion strength (most repulsion to least) is:
• Lone pair - lone pair.
• Lone pair - bonding pair.
• Bonding pair - bonding pair.

## Electron Pairs and Geometry

There is an optimal geometry for every number of electron pairs. The different types of geometry are:

### Linear

• If the central atom of a molecule has two electron pairs, it will likely adopt a linear geometry.
• The bond angle will be 180o.

### Trigonal planar

• If the central atom of a molecule has three electron pairs, it will likely adopt a trigonal planar geometry.
• The bond angle will be 120o.

### Tetrahedral

• If the central atom of a molecule has four electron pairs, it will likely adopt a tetrahedral geometry.
• The bond angle will be 109.5o.

### Trigonal bipyramidal

• If the central atom of a molecule has five electron pairs, it will likely adopt a trigonal bipyramidal geometry.
• Molecules with this shape have two bond angles:
• A 120o angle around the equator.
• A 90o angle from equator to apex.

### Octahedral

• If the central atom of a molecule has six electron pairs, it will likely adopt an octahedral geometry.
• The bond angle will be 90o.