Shapes of molecules
In a nutshell
The shape of a molecule depends on the electron pair arrangement around the central atom. The number of pairs of electrons around the central atom influences the arrangement.
Electron pair repulsions
Electrons repel each other in an atom.
The amount of repulsion will depend on the type of electron pair:
- Lone pair/lone pair has the largest repulsion angle
- Lone pair/bonded pair has the second largest repulsion angle
- Bonded pair/bonded pair has the smallest repulsion angle
The shape of a molecule will depend on the number of electron pairs as well as the type of electron pair.
Electron pair repulsion theory
You can predict the shape of a molecule using electron pairs.
procedure
1. | First, find the central atom. |
2. | Work out the number of outer shell electrons in the central atom using either the periodic table or a dot and cross diagram. |
3. | Use the molecular formula to find the number of electrons that are shared with the central atom. |
4. | Count the the number of electrons and divide by two to find the number of electron pairs. Tip: Remember the charges on ions. |
5. | Compare the number of electron pairs with the number of bonds to find any lone pairs. Tip: A double bond will count as two bonds. A triple bond will count as three bonds. |
6. | Use the number of electron pairs, number of lone pairs and number of bonding centres around the central atom to find the shape of the molecule. |
Shapes of molecules
There are several different shapes that molecules can adopt.
shape | electron pairs around central atom | bond angles | example |
Linear | Two electrons pairs | | |
Trigonal planar | Three electron pairs No lone pairs | | |
Non-linear/bent | Three electron pairs One lone pair | | |
Tetrahedral | Four electron pairs No lone pairs | 109.5° | |
Trigonal pyramidal | Four electron pairs One lone pair | 106.7° | |
Non-linear/bent | Four electron pairs Two lone pairs | 104.5° | |
Trigonal bipyramidal | Five electrons pairs No lone pairs | | |
Seesaw | Five electron pairs One lone pair | | |
Distorted T | Five electron pairs Two lone pairs | | |
Octahedral | Six electron pairs No lone pairs | | |
Square pyramidal | Six electron pairs One lone pair | | |
Square planar | Six electron pairs Two lone pairs | | |