Draw the best Lewis structure for Cl3⁻ and determine the formal charge on all 3 Cl atoms.

Draw the best Lewis structure for Cl3⁻ and determine the formal charge on all 3 Cl atoms.

The Correct Answer and Explanation is:

ChatGPT said:

Lewis Structure for Cl₃⁻ (Trichloride Anion)

The best Lewis structure for the trichloride anion (Cl₃⁻) is:

rubyCopyEdit     :Cl:
      |
:Cl–Cl:
      |
     :Cl:

But more precisely and clearly (with lone pairs):

rubyCopyEdit   ..
:Cl:–Cl:–:Cl:
   ..  ..  ..

Each outer Cl atom has 3 lone pairs, and the central Cl has 3 lone pairs plus single bonds to each of the two terminal Cl atoms. The negative charge is delocalized over the structure, often represented as a formal charge on the central atom.

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Step-by-Step Explanation

1. Count Total Valence Electrons:
   • Each Cl atom has 7 valence electrons.
   • There are 3 Cl atoms → 3 × 7 = 21 electrons.
   • Plus 1 extra electron due to the negative charge → 22 valence electrons total.
2. Skeleton Structure:
   • Place one Cl atom in the center bonded to two others: Cl–Cl–Cl.
3. Assign Electrons:
   • Form single bonds between atoms (2 bonds × 2 e⁻ = 4 e⁻).
   • Distribute remaining 18 electrons to satisfy octets on terminal atoms first.
4. Lone Pairs:
   • Each outer Cl needs 6 more electrons (3 lone pairs) to complete the octet.
   • That uses 12 electrons (6 for each terminal Cl).
   • Remaining 6 electrons go to the central Cl as 3 lone pairs.
5. Formal Charges:
   • Formal charge = Valence – (Lone pairs + ½ Bonding)
     • Terminal Cl atoms: 7 – (6 + 1) = 0
     • Central Cl atom: 7 – (6 + 1) = 0
   • But we have an extra electron (22 total), so the negative charge is distributed as a formal charge of –1 on the central Cl.

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Final Formal Charges:

• Central Cl: –1
• Each terminal Cl: 0

This structure satisfies the octet rule and reflects the delocalization of the extra electron over the central atom. Cl₃⁻ is also known to be linear in geometry and exists in some salts like CsCl₃.