Introduction

Why is diamond one of the hardest substances known while graphite, its own allotrope of pure carbon, is soft enough to write with? Why do hydrogen and oxygen exist as diatomic gas molecules while the noble gases stay stubbornly monoatomic? Every one of these puzzles comes down to the same underlying question -- how do atoms actually hold together -- and that question is what this unit answers.

What this unit sets out to do. By the end of it you should be able to:

• describe the Kossel-Lewis approach to chemical bonding, and explain the octet rule;
• sketch the Lewis structures of simple molecules, and describe the formation of different types of bonds along with bond parameters;
• sketch resonance structures for simple molecules;
• apply electronegativity to explain the polarity of covalent bonds;
• describe VSEPR theory and use it to predict the shapes of simple molecules;
• explain the valence bond approach to covalent bond formation;
• explain the different types of hybridisation involving s, p and d orbitals, and sketch the shapes of the resulting simple covalent molecules;
• explain molecular orbital theory, calculate bond order, and explain the magnetic properties of H2, O2, N2, CO and NO;
• briefly describe metallic bonding.