Structure and Bonding

4.1

Structure of simple molecules using the qualitative model of repulsion between electron pairs.

• Only molecules wit up to four electron pairs around a central atom should be considered. The qualitative effect of lone pairs on structure should be treated. A simple non-mathematical treatment of tetrahedral carbon is required. Hybridization is not required and will not be examined.

4.2 Structure of Solids

Use of molar heats of vaporization and boiling points of elements and compounds to indicate the existence of two main structural types: strong (giant) structures and weak (molecular) structures.

Types of Structures:

(a) Sodium chloride as an example of a giant ionic structure. Lattice energy for ionic compounds and the processes of melting with reference to appropriate energy level diagrams. The relationship between heat of solution, lattice energy and hydration energy. The heats of solution of an anhydrous and a hydrated salt should be compared.

(b) Diamond as an example of a giant covalent structure and graphite as an example of a layer structure. Allotrophy, illustrated by the different structures and properties of graphite and diamond.

(c) Copper as an example of a giant metallic structure.

(d) Iodine as an example of a giant molecular structure.

• The different structures should be treated comparatively to bring out the influence of structure on physical properties. The strength of the covalent bond should be emphasized. Other substances chosen from the elements and compounds covered in Section 8 should be used to illustrate this section.

4.3 Chemical Bonding

The covalent bond: the nature and strength of the covalent bond.

(a) Existence of polar and non-polar covalent bonds; ionic bonds as extreme examples of polar covalent bonds. Co-ordinate covalent bonds, c.f. NH3 and NH4+ . *The point should be made clearly that the co-ordinate bond is not different in nature from the normal covalent bond.

(b) Effect of polarity of the solvent on solubility should be mentioned.

(c) Bond energies: the use of enthalpies of combustion and formation in calculating bond energies should be mentioned.

Ionization energy and electron affinity: relation to the type of bonding in compounds; electro negativity.

• Electro negativity values should not be quoted but a qualitative idea that different elements have different tendencies to lose or attract electrons should be mentioned. Trends across and down the Periodic Table should be brought out.

The hydrogen bond: effect on structure and physical properties. Van der Waal's bonds; molecular interactions in gases, liquids and solids.

Delocalized electrons

(a) The metallic bond.

(b) Graphite as an example of a layer lattice with delocalized electrons (c.f. metallic character).

(c) Benzene as an example of a molecule with delocalized electrons.