1.40     explain, using dot and cross diagrams, the formation of covalent compounds by electron sharing for the following substances:

i     hydrogen*

ii    chlorine

iii   hydrogen chloride*

iv   water*

v    methane*

vi   ammonia*

vii oxygen*

viii nitrogen

ix   carbon dioxide

x    ethane

xi   ethene

*Asterisked examples are shown below. You need to learn to draw all of the molecules in the list equally clearly. Note the double bond in oxygen. Nitrogen has a triple bond. Find out how to draw each of the others and practise them thoroughly.