Advanced Higher Advanced Higher ChemistryChemistry

Unit 1 Unit 1

Shapes of molecules and Shapes of molecules and polyatomic ionspolyatomic ions

Electron RepulsionElectron Repulsion

• The shapes of molecules are The shapes of molecules are determined by the numbers of determined by the numbers of bonding and non-bonding (lone) bonding and non-bonding (lone) electron pairs.electron pairs.

• There is repulsion between the There is repulsion between the orbitals as a result of the electrons.orbitals as a result of the electrons.

• Repulsive effect :Repulsive effect :

bonded:bonded < bonded:lone < lone:lonebonded:bonded < bonded:lone < lone:lone

Electron RepulsionElectron Repulsion

• The shape The shape adopted by the adopted by the molecule is the molecule is the shape which has shape which has the minimum the minimum repulsion.repulsion.

• The electron The electron pairs are as far pairs are as far apart as apart as possible.possible.

Linear MoleculesLinear Molecules

• There are two electron There are two electron pairs involved in pairs involved in bonding.bonding.

• The electron pairs lie The electron pairs lie as far apart as possible as far apart as possible resulting in a linear resulting in a linear molecule.molecule.

Trigonal Planar MoleculesTrigonal Planar Molecules• There are three pairs of electrons There are three pairs of electrons

involved in bonding.involved in bonding.• Repulsion is minimised by the Repulsion is minimised by the

bonding pairs lying flat at 120bonding pairs lying flat at 120oo..

Tetrahedral MoleculesTetrahedral Molecules

• When there are four bonding pairs When there are four bonding pairs of electrons the shape is of electrons the shape is tetrahedral.tetrahedral.

• The bond angle is 109.5The bond angle is 109.5oo..

Tetrahedral MoleculesTetrahedral Molecules

• If the lone pair of electrons in If the lone pair of electrons in ammonia forms a dative covalent ammonia forms a dative covalent bond, a tetrahedral ammonium ion bond, a tetrahedral ammonium ion (NH(NH44

++) is formed.) is formed.

• Water also has a tetrahedral Water also has a tetrahedral arrangement of electron pairs but the arrangement of electron pairs but the molecule is not tetrahedral.molecule is not tetrahedral. Note the

decreasing angle L to R

Pyramidal MoleculesPyramidal Molecules

• Four filled orbitals, Four filled orbitals, made up of one made up of one lone pair and three lone pair and three bonded pairs, bonded pairs, results in a results in a pyramidal shape.pyramidal shape.

• The bonded pair The bonded pair angle is 107angle is 107oo as the as the lone pair pushes lone pair pushes the bonded pairs the bonded pairs closer together.closer together.

V-shaped/Bent MoleculesV-shaped/Bent Molecules

• In a water molecule, there are two In a water molecule, there are two bonded pairs and two lone pairs.bonded pairs and two lone pairs.

• The lone pairs repel the bonded The lone pairs repel the bonded pairs producing a bent molecule pairs producing a bent molecule with a bond angle of 105with a bond angle of 105oo..

Trigonal Bipyramidal Trigonal Bipyramidal MoleculesMolecules

• Trigonal Trigonal bipyramidal bipyramidal molecules are molecules are formed when formed when there are five there are five filled bonding filled bonding pairs of electrons.pairs of electrons.

:: :

T-shaped moleculesT-shaped molecules

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

Octahedral MoleculesOctahedral Molecules

• Six bonding pairs Six bonding pairs results in an results in an octahedral molecule.octahedral molecule.

• If the molecule were a If the molecule were a solid shape it would solid shape it would be an octahedron.be an octahedron.

p32

Square pyramidal Square pyramidal moleculesmolecules

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

Square planar moleculesSquare planar molecules

• Square planar Square planar molecules are molecules are formed when formed when there are four there are four bonding pairsbonding pairs

and two lone pairs and two lone pairs of electrons.of electrons.

ExerciseExercise

Now try the exercise on page 13 of Now try the exercise on page 13 of your notes.your notes.