Hydrogen atoms have one electron each. Chlorine atoms have seven outer-shell electrons.
Draw a dot-and-cross diagram of a hydrogen molecule, H2\mathrm{H_2}H2. Use a dot for the electron from one atom and a cross for the electron from the other.
Draw a dot-and-cross diagram of hydrogen chloride, HCl\mathrm{HCl}HCl. Show all outer-shell electrons, using a cross for hydrogen's electron and dots for chlorine's electrons.
Explain how sharing electrons gives each hydrogen atom a complete first shell.
53 exam-style questions on Edexcel GCSE Chemistry 2.4 Covalent bonding, covering 2.4.1 Covalent bonding and simple molecules. Each one has a worked solution and a mark scheme showing where the marks go.