3e. Covalent bonding of chlorine molecule Cl₂

Covalent bonding diagram for CHLORINE covalent molecule, molecular formula Cl₂

Two chlorine atoms (2.8.7) form the molecule of the element chlorine Cl₂

Chlorine, a Cl atom, is one electron short of a full outer shell of 8 like argon (2.8.8), so two chlorine atoms share an electron to have full outer shells.

 and combine to form where both atoms have a pseudo argon structure of 8 outer electrons around each atom. The electronic dot & cross Lewis diagrams for covalent bonding in the diatomic molecule chlorine - outer shell electrons only.

(Lewis diagram of chlorine on the left) simplified 'dot and cross' electronic diagram for the covalently bonded chlorine molecule

The chlorine molecule is held together by the strong chlorine–chlorine single covalent bond from sharing outer electrons,  Cl–Cl (displayed formula).

Electronically, both chlorines (2.8.7) become like argon (2.8.8), so the chlorine atoms effectively have a full outer shell in forming the covalent bonds when the atoms share their outer electrons.

All the other halogens would be similar e.g. F₂, Br₂ and I₂ etc. The Venn style diagram on the right is the best style for chlorine, clearly showing the sharing of the pairs of electrons for the single covalent bonds in the chlorine molecule (in a sort of Venn diagram style).

Here the valency of halogens like chlorine is 1.

Note that the two inner shells of chlorine's electrons are not shown some diagrams above, but they are in the diagram on the left, and, remember, 'dots' and 'crosses' are all the same electrons in their specific energy levels and only the outer shells of electrons are involved in the covalent bonding here.

 This the full 'dot and cross' electronic diagram for the covalent bonding in the chlorine molecule.

 

   AND 

ball and stick diagram for chlorine  AND  space-filling model of chlorine

 

Bromine and iodine

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

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

Br₂  Br-Br   and   I₂ I-I

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

Comments

Melting point of chlorine -101 ^oC

Boiling point of chlorine -34 ^oC

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

Chlorine is a poisonous-toxic green gas at room temperature

Extra information about Chlorine for e.g. GCSE/IGCSE level Chemistry Revision Notes

Properties of Chlorine

• Symbol: Cl
• Atomic number: 17
• Molecular form: Cl₂ (diatomic molecule)
• Physical properties:
  • Greenish-yellow gas
  • Strong, choking smell
  • Denser than air
  • Moderately soluble in water
• Chemical properties:
  • Toxic and corrosive
  • Powerful oxidising agent
  • Reacts with hydrogen → hydrogen chloride (HCl)
  • Reacts with metals → metal chlorides
  • Reacts with water → mixture of hydrochloric acid (HCl) and hypochlorous acid (HOCl)
  • A member of the group 7 halogen elements of the periodic table

Laboratory Preparation

• Method: Oxidising concentrated hydrochloric acid
  • Example: MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂↑
• Collection: By downward delivery (chlorine is denser than air)
• Test for chlorine:
  • Bleaches damp litmus paper (turns red → white)
  • Distinctive greenish-yellow colour and choking smell

Industrial Manufacture

• Chlor-alkali process (electrolysis of brine):
  • Brine = concentrated sodium chloride solution
  • Products:
    • Chlorine gas at the anode
    • Hydrogen gas at the cathode
    • Sodium hydroxide solution
• Balanced equation:
  • 2NaCl + 2H₂O → Cl₂ + H₂ + 2NaOH

Uses of Chlorine

• Water treatment: Disinfecting drinking water and swimming pools
• Bleaching: Paper, textiles
• Chemicals manufacture:
  • PVC (polyvinyl chloride)
  • Hydrochloric acid
  • CFCs (historically, now restricted)
• Household products: Disinfectants, bleach
• Organic chemistry: Chlorination of hydrocarbons

Typical Exam Board Requirements

Student Exam Tips

• Always state Cl₂, not Cl when referring to chlorine gas.
• Test for chlorine: Mention bleaching of damp litmus paper (turns red → white).
• Industrial manufacture: Be precise—chlor-alkali process, not just “electrolysis.”
• Equation accuracy: Balance equations carefully (common exam mark lost here).
• Collection method: Downward delivery (chlorine is denser than air).
• Uses: Link to syllabus keywords (water treatment, PVC, bleach).

Common Misconceptions about chlorine

•  Thinking chlorine is collected by upward delivery (it’s denser than air, so downward delivery is correct).
•  Believing chlorine is harmless in water treatment (it is toxic, but used in controlled amounts).
•  Confusing chlorine with chlorides (chlorine = Cl₂ gas, chlorides = salts like NaCl).
•  Forgetting chlorine is diatomic (Cl₂, not Cl).
•  Assuming chlorine only makes hydrochloric acid when dissolved in water (it also forms hypochlorous acid, responsible for bleaching/disinfecting action).

Halogen links

Group 7 The Halogens, physical & chemical properties

See also Salt NaCl extraction, electrolysis products, uses of halogens

and Quizzes-Worksheets section 8. (opens in new window for convenience)

and Electrolysis of sodium chloride solution (brine)