Revision:Shapes of Molecules and Ions

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Electron Pair Repulsion Theory

The electron pair repulsion theory (Also known as the VSEPR theory) predicts the shapes of molecules and ions based on the number of atoms bonded to the central atom plus the number of unbonded lone pairs of electrons around it. The shape of the molecule or ion can then be predicted. Wikipedia has an excellent article on this. Specifically (For Edexcel new specification), the shapes of the following molecules are needed:

The bond angle in molecules or ions is the angle made between two adjacent pairs of electrons or atoms if they were represented by a straight line. The bond length is the distance between the nuclei of the atoms in the bond.

Carbon

Carbon exists as Allotropes. Allotropes are different forms of the same element that exist in the same physical state, the differences between the allotropes arise due to the different ways of bonding with the atoms thus they have different molecular shapes. Carbon has two main Allotropes: Diamond and Graphite

Diamond Each C atom is making 4 sigma bonds to 4 other C atoms This results in a tetrahedral structure with bond angles of 109.5 degrees.

Graphite Each C atom is maing 3 sigma bonds to 3 other C atoms. The fourth electron is delocalised between layers of hexagonal rings. It is a trigonal planar structure with bond angles of 120 degrees.

Note- Both of the above are giant network covalent structures

Properties In diamond (Diagram needed), the carbon atoms form a giant covalent lattice. It is one of the hardest known materials, and is very good at refracting light. It is mainly used as gemstones in jewellery.

Carbon atoms can arrange in hexagonal layers - the structure of graphite. Because each carbon atom only has 3 bonds to it, there is a spare electron from each carbon which allows graphite to conduct electricity. Graphite is used to make steel and in pencils.

Fullerenes are where carbon atoms arrange themselves in a spherical shape. With other atoms, fullerenes can be superconductive at low temperatures.

Carbon nanotubes are carbon atoms arranged in a cylindrical shape. They have the potential of carrying drugs into cells.