1. Advanced Higher Chemistry Unit 1 Shapes of molecules and polyatomic ions

2. Electron Repulsion The shapes of molecules are determined by the numbers of bonding and non-bonding (lone) electron pairs. The shapes of molecules are determined by the numbers of bonding and non-bonding (lone) electron pairs. There is repulsion between the orbitals as a result of the electrons. There is repulsion between the orbitals as a result of the electrons. Repulsive effect : Repulsive effect : bonded:bonded < bonded:lone < lone:lone

3. Electron Repulsion The shape adopted by the molecule is the shape which has the minimum repulsion. The shape adopted by the molecule is the shape which has the minimum repulsion. The electron pairs are as far apart as possible. The electron pairs are as far apart as possible.

4. Linear Molecules There are two electron pairs involved in bonding. There are two electron pairs involved in bonding. The electron pairs lie as far apart as possible resulting in a linear molecule. The electron pairs lie as far apart as possible resulting in a linear molecule.

5. Trigonal Planar Molecules There are three pairs of electrons involved in bonding. There are three pairs of electrons involved in bonding. Repulsion is minimised by the bonding pairs lying flat at 120 o. Repulsion is minimised by the bonding pairs lying flat at 120 o.

6. Tetrahedral Molecules When there are four bonding pairs of electrons the shape is tetrahedral. When there are four bonding pairs of electrons the shape is tetrahedral. The bond angle is 109.5 o. The bond angle is 109.5 o.

7. Tetrahedral Molecules If the lone pair of electrons in ammonia forms a dative covalent bond, a tetrahedral ammonium ion (NH 4 + ) is formed. If the lone pair of electrons in ammonia forms a dative covalent bond, a tetrahedral ammonium ion (NH 4 + ) is formed. Water also has a tetrahedral arrangement of electron pairs but the molecule is not tetrahedral. Water also has a tetrahedral arrangement of electron pairs but the molecule is not tetrahedral. Note the decreasing angle L to R

8. Pyramidal Molecules Four filled orbitals, made up of one lone pair and three bonded pairs, results in a pyramidal shape. Four filled orbitals, made up of one lone pair and three bonded pairs, results in a pyramidal shape. The bonded pair angle is 107 o as the lone pair pushes the bonded pairs closer together. The bonded pair angle is 107 o as the lone pair pushes the bonded pairs closer together.

9. V-shaped/Bent Molecules In a water molecule, there are two bonded pairs and two lone pairs. In a water molecule, there are two bonded pairs and two lone pairs. The lone pairs repel the bonded pairs producing a bent molecule with a bond angle of 105 o. The lone pairs repel the bonded pairs producing a bent molecule with a bond angle of 105 o.

10. Trigonal Bipyramidal Molecules Trigonal bipyramidal molecules are formed when there are five filled bonding pairs of electrons. Trigonal bipyramidal molecules are formed when there are five filled bonding pairs of electrons. :: :

11. T-shaped molecules T-shaped molecules are formed when there are three bonding pairs and two lone pairs of electrons. T-shaped molecules are formed when there are three bonding pairs and two lone pairs of electrons.

12. Octahedral Molecules Six bonding pairs results in an octahedral molecule. Six bonding pairs results in an octahedral molecule. If the molecule were a solid shape it would be an octahedron. If the molecule were a solid shape it would be an octahedron. p32

13. Square pyramidal molecules Square pyramidal molecules are formed when there are five bonding pairs and one lone pair of electrons. Square pyramidal molecules are formed when there are five bonding pairs and one lone pair of electrons.

14. Square planar molecules Square planar molecules are formed when there are four bonding pairs Square planar molecules are formed when there are four bonding pairs and two lone pairs of electrons. and two lone pairs of electrons.

15. Exercise n Now try the exercise on page 13 of your notes.