Introduction

Two everyday comparisons open this unit. Diamond (an allotrope of carbon) is very hard, while graphite, another allotrope of the very same element, is very soft. Hydrogen and oxygen gas are diatomic (H₂, O₂) while the inert (noble) gases are monoatomic. Carbon combines with chlorine to give carbon tetrachloride, a liquid that is insoluble in water; sodium combines with chlorine to give sodium chloride, a hard, brittle, water-soluble solid. All of these contrasts trace back to the same underlying question: what kind of interaction holds the atoms or ions of a substance together?

The interatomic attractive forces that hold the constituent atoms or ions of a substance together are called chemical bonds. A second, related question this unit answers is why atoms combine only in certain fixed combinations and not others -- oxygen combines with hydrogen to give water (H₂O, a 'V'-shaped/bent molecule) but with carbon to give carbon dioxide (CO₂, a linear molecule). Why do the same two elements sometimes combine in a 1:2 ratio and sometimes not, and why do the resulting molecules have such different shapes? …