Advanced Higher Chemistry Unit 1 Lewis diagrams and resonance structures

Lewis Diagrams Lewis electron-dot diagrams show the arrangement of electrons within a covalently bonded molecule. Lewis electron-dot diagrams show the arrangement of electrons within a covalently bonded molecule. One atom’s electrons are represented by dots and the other atoms by crosses. One atom’s electrons are represented by dots and the other atoms by crosses.

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

Dative Covalent Bonds When both electrons in a covalent bond are from the same atom it is known as a dative covalent bond. When both electrons in a covalent bond are from the same atom it is known as a dative covalent bond. e.g. the formation of ammonium from ammonia and H + : e.g. the formation of ammonium from ammonia and H + :

n The hydrogen ion has no electrons (it has a single positive charge) and both electrons for the dative covalent bond come from the nitrogen atom’s lone pair. n A dative covalent bond is the same as any other covalent bond i.e. in the ammonium ion the four N-H bonds are all identical even though they have been formed differently. n The ion charge is spread over the whole ion i.e. the ion is delocalised.

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

Resonance structures n Sometimes it is possible to draw more than one equivalent bond diagram for a structure. n These are known as resonance structures

Ozone (O 3 ) The O-O bonds in ozone are of equal length but the Lewis dot diagrams suggest a double and single bond are present. These bonds differ in length. The O-O bonds in ozone are of equal length but the Lewis dot diagrams suggest a double and single bond are present. These bonds differ in length. There are two possible resonance structures for ozone however neither gives an satisfactory representation. There are two possible resonance structures for ozone however neither gives an satisfactory representation.

The actual bonding is between the two resonance structures. The actual bonding is between the two resonance structures. There is a single structure with two of the bonding electrons delocalised symmetrically across all three oxygen atoms. There is a single structure with two of the bonding electrons delocalised symmetrically across all three oxygen atoms. LT p28

the carbonate ion (CO 3 2- ) n There are three resonance structures of the carbonate ion:

Exercise n Draw resonance structures for the nitrate ion (NO 3 - ), sulphur dioxide (SO 2 ) and sulphur trioxide (SO 3 ).