3f. The covalent bonding in the hydrogen chloride molecule HCl

Covalent bonding diagram for HYDROGEN CHLORIDE covalent molecule, molecular formula HCl

One atom of hydrogen (1) combines with one atom of chlorine (2.8.7) to form the molecule of the compound hydrogen chloride HCl

Both hydrogen and chlorine have one electron short of a full outer shell (2 for H, 8 for Cl), so both atoms share an electron to have full outer shells.

 and combine to form where hydrogen is electronically like helium (2) and chlorine like argon (2.8.8).

The hydrogen chloride molecule is held together by the strong hydrogen–chlorine single covalent bond by sharing electrons, H–Cl (displayed formula). Note that the two inner shells of chlorine's electrons (2.8.7) are NOT shown (see chlorine atom diagram in example 2.

The Venn style diagram on the right is the best style for the diatomic molecule hydrogen chloride, clearly showing the sharing of the pairs of electrons for the single covalent bonds in the hydrogen chloride molecule (in a sort of Venn diagram style).

Electronically, hydrogen (1) becomes like helium (2) and chlorine (2.8.7) becomes like argon (2.8.8), so the hydrogen and chlorine atoms effectively have full outer shells in forming the covalent bonds when the atoms share their outer electrons. The two inner shells of chlorine's electrons are not shown, only the outer shells of electrons are involved in the covalent bonding here.

(Lewis diagram of hydrogen chloride) simplified 'dot and cross' electronic diagram for the covalently bonded hydrogen chloride molecule.

  is the full 'dot and cross' electronic diagram for the covalent bonding in the hydrogen chloride molecule - all the electrons are shown.

The electronic dot & cross Lewis diagrams for covalent bonding in hydrogen chloride.

All the other hydrogen halides will be similar e.g. hydrogen fluoride HF, hydrogen bromide HBr and hydrogen iodide HI.

  AND

ball and stick model of hydrogen chloride AND  space-filling model of hydrogen chloride

Note:

Hydrogen chloride gas is a true covalent substance consisting of small HCl molecules.

If the gas is dissolved in a hydrocarbon solvent like hexane or methylbenzene it remains as covalent HCl molecules and because there are no ions present, the solution does not conduct electricity.

However, if hydrogen chloride gas is dissolved in water, things are very different and the HCl molecules split into ions.

Hydrochloric acid is formed which consists of a solution of hydrogen ions (H⁺) and chloride ions (Cl^–).

The solution then conducts electricity and passage of a d.c. current causes electrolysis to take place forming hydrogen and chlorine.

Hydrogen bromide and hydrogen iodide

(also group 7 halogen halides and another example of a group pattern in the periodic table)

In terms of the outer electrons, the electronic diagrams are identical for hydrogen bromide and hydrogen iodide i.e. a single covalent bond joins the two halogen atoms together.

HBr  H-Br   and   HI H-I

In all the above diagrams you can swap Cl with a Br or an I atom.

Comments

Melting point of hydrogen chloride -114 ^oC

Boiling point of hydrogen chloride -85 ^oC

You would expect low values because of the weak intermolecular forces between small covalent molecules like chlorine.

Hydrogen chloride is a very acidic choking colourless gas at room temperature.

Don't confuse HCl(g) gas with HCl(aq) hydrochloric acid solution.

Reminder: How to work out formula of covalent compounds without going through some demanding electronic thinking is described on the "Elements, Compounds and Mixtures" page and it is followed by a section on naming compounds.

Extra information about Hydrogen Chloride Gas e.g. for GCSE/IGCSE Level Chemistry Revision Notes

Properties of hydrogen chloride

• Formula: HCl
• Molecular form: Covalent, diatomic molecule (H–Cl)
• Physical properties:
  • Colourless gas
  • Sharp, pungent smell
  • Denser than air
  • Highly soluble in water → forms hydrochloric acid (HCl(aq))
• Chemical properties:
  • Acidic when dissolved in water (strong acid, fully ionises: HCl → H⁺ + Cl^-)
  • Reacts with ammonia gas → forms dense white fumes of ammonium chloride (NH₄Cl)

Laboratory Preparation

• Method: React concentrated hydrochloric acid with a solid ionic chloride and a dehydrating agent
  • Example: NaCl + H₂SO₄(conc.) → NaHSO₄ + HCl↑
• Collection: By downward delivery (denser than air, soluble in water so cannot be collected over water)
• Tests:
  • Turns damp blue litmus paper red (acidic gas)
  • Forms white fumes with ammonia gas

Industrial Manufacture

• Chlor-alkali industry (by-product):
  • Electrolysis of brine produces chlorine, hydrogen, and sodium hydroxide → hydrogen and chlorine can be combined to make HCl
  • See Electrolysis of sodium chloride solution (brine)
• Direct synthesis:
  • H₂ + Cl₂ → 2HCl (exothermic reaction, controlled conditions)
• Other processes:
  • As a by-product in organic chlorination reactions

Uses of Hydrogen Chloride

• Hydrochloric acid manufacture: Dissolving HCl gas in water
• Chemical industry: Production of PVC, dyes, pharmaceuticals
• Laboratory reagent: Acidification, preparation of chlorides
• Food industry: Hydrochloric acid used in processing (E507 additive)
• Testing for ammonia gas: White fumes of ammonium chloride

Typical Exam Board Requirements

Student Exam Tips

• Always distinguish between HCl (gas) and HCl(aq) (hydrochloric acid).
• Collection method: Downward delivery (denser than air, soluble in water).
• Tests: Damp blue litmus → red; ammonia gas → white fumes.
• Industrial manufacture: Remember direct synthesis (H₂ + Cl₂ → 2HCl).
• Equations: Balance carefully, especially in lab prep reactions.
• Exam wording: “Hydrogen chloride gas dissolves in water to form hydrochloric acid” — don’t confuse the two.

Common Misconceptions

•  Thinking hydrogen chloride gas is hydrochloric acid (it only becomes hydrochloric acid when dissolved in water).
•  Assuming HCl can be collected over water (it is highly soluble, so this is incorrect).
•  Forgetting HCl is covalent in the gas phase (students sometimes assume ionic because it forms ions in solution).
•  Confusing “test for hydrogen chloride” with “test for hydrochloric acid” — litmus test is for the gas, not the solution.
•  Believing HCl gas is harmless (it is corrosive and toxic).