O-Level Chemical Bonding and Structure: worked solution

Question

When ammonium chloride breaks down, hydrogen chloride gas is produced. Using a dot-and-cross diagram, show how the atoms in hydrogen chloride are bonded, drawing only the outer-shell (valence) electrons.

Hydrogen chloride, HCl, is a simple covalent molecule. Hydrogen has 1 outer electron; chlorine has 7 outer electrons. They share one pair of electrons to form a single covalent bond. This gives hydrogen a share in 2 electrons (like helium) and chlorine a share in 8 electrons (a full outer shell). Dot-and-cross diagram (outer electrons only): H shows its single electron as a cross (x); Cl shows its 7 outer electrons as dots. The shared pair sits between H and Cl: one dot from Cl and one cross from H (x .). Cl keeps its other 3 lone pairs of dots around it: . . .. Cl (x.) H . . So: one shared pair (x from H, . from Cl) between the two atoms, and three lone pairs (6 dots) on the Cl.