Shapes of Molecules

-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.

• 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.

• 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.

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

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

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

• 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 120^o angle around the equator.
• A 90^o angle from equator to apex.

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