Advanced Higher Advanced Higher ChemistryChemistry

Unit 1 Unit 1

Lewis diagrams and Lewis diagrams and

resonance structuresresonance structures

Lewis DiagramsLewis Diagrams

• Lewis electron-dot diagrams show the Lewis electron-dot diagrams show the arrangement of electrons within a arrangement of electrons within a covalently bonded molecule.covalently bonded molecule.

• One atom’s electrons are represented One atom’s electrons are represented by dots and the other atoms by crosses.by dots and the other atoms by crosses.

Lone Pairs of electronsLone Pairs of electrons

As can be seen in the previous diagram, not all pairs of electrons are involved in bonding. These non-bonding pairs are known as lone pairs

e.g. in the oxygen molecule there are two bonding pairs and four lone pairs and in the nitrogen molecule there are three bonding pairs and two lone pairs.

Polyatomic ionsPolyatomic ions

For polyatomic ions e.g. NHFor polyatomic ions e.g. NH44++, , SOSO442-,2-, etc. electrons must be added etc. electrons must be added or gained depending on the or gained depending on the charge.charge.Now try the exercise on page 4 of your notes.

Dative Covalent BondsDative Covalent Bonds

• When both electrons in a covalent bond When both electrons in a covalent bond are from the same atom it is known as a are from the same atom it is known as a dative covalent bond.dative covalent bond.

e.g. the formation of ammonium from e.g. the formation of ammonium from ammonia and Hammonia and H++ : :

The hydrogen ion has no electrons (it has The hydrogen ion has no electrons (it has a single positive charge) and both a single positive charge) and both electrons for the dative covalent bond electrons for the dative covalent bond come from the nitrogen atom’s lone pair.come from the nitrogen atom’s lone pair.

A dative covalent bond is the same as any A dative covalent bond is the same as any other covalent bond i.e. in the ammonium other covalent bond i.e. in the ammonium ion the four N-H bonds are all identical ion the four N-H bonds are all identical even though they have been formed even though they have been formed differently.differently.

The ion charge is spread over the whole The ion charge is spread over the whole ion i.e. the ion is delocalised.ion i.e. the ion is delocalised.

The ion can be The ion can be represented by represented by Lewis diagrams or Lewis diagrams or by equivalent by equivalent bond diagrams:bond diagrams:

Resonance structuresResonance structures

Sometimes it is possible to draw Sometimes it is possible to draw more than one equivalent bond more than one equivalent bond diagram for a structure.diagram for a structure.

These are known as resonance These are known as resonance structuresstructures

Ozone (OOzone (O33))

• The O-O bonds in ozone are of equal The O-O bonds in ozone are of equal length but the Lewis dot diagrams length but the Lewis dot diagrams suggest a double and single bond are suggest a double and single bond are present. These bonds differ in length.present. These bonds differ in length.

• There are two possible resonance There are two possible resonance structures for ozone however neither structures for ozone however neither gives an satisfactory representation.gives an satisfactory representation.

• The actual bonding is between the The actual bonding is between the two resonance structures.two resonance structures.

• There is a single structure with two There is a single structure with two of the bonding electrons of the bonding electrons delocalised symmetrically across delocalised symmetrically across all three oxygen atoms.all three oxygen atoms.

LT p28

the carbonate ion (COthe carbonate ion (CO332-2-))

There are three resonance There are three resonance structures of the carbonate ion:structures of the carbonate ion:

ExerciseExercise

Draw resonance structures for the Draw resonance structures for the nitrate ion (NOnitrate ion (NO33--), sulphur dioxide ), sulphur dioxide (SO(SO22) and sulphur trioxide (SO) and sulphur trioxide (SO33).).