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Question: number of localised lp in $CCl_3^-$....

number of localised lp in CCl3−CCl_3^-.

Answer

10

Explanation

Solution

To determine the number of localized lone pairs (lp) in the CCl3−CCl_3^- ion, we first need to draw its Lewis structure.

  1. Calculate the total number of valence electrons:

    • Carbon (C) is in Group 14, so it has 4 valence electrons.
    • Chlorine (Cl) is in Group 17, so each Cl atom has 7 valence electrons. There are 3 Cl atoms, contributing 3×7=213 \times 7 = 21 valence electrons.
    • The negative charge (-) indicates an additional electron.
    • Total valence electrons = 4(from C)+21(from 3 Cl)+1(from charge)=264 (\text{from C}) + 21 (\text{from 3 Cl}) + 1 (\text{from charge}) = 26 electrons.

  2. Draw the Lewis structure:

    • Carbon is the central atom, bonded to three chlorine atoms. This uses 3×2=63 \times 2 = 6 electrons for the three single bonds.
    • Remaining electrons = 26−6=2026 - 6 = 20 electrons.
    • Distribute these remaining electrons as lone pairs around the chlorine atoms to satisfy their octets. Each chlorine atom needs 6 more electrons (3 lone pairs). This uses 3×6=183 \times 6 = 18 electrons.
    • Remaining electrons = 20−18=220 - 18 = 2 electrons.
    • Place these last 2 electrons as a lone pair on the central carbon atom to satisfy its octet.
    • The Lewis structure shows carbon bonded to three chlorine atoms, with each chlorine atom having 3 lone pairs, and the carbon atom having 1 lone pair.

    The structure is: :Cl: | :Cl: - C - :Cl: | :lp:

  3. Determine localization of lone pairs:

    • Lone pairs on Chlorine atoms: Each chlorine atom has 3 lone pairs. These lone pairs are held closely by the electronegative chlorine atoms and are not involved in delocalization through resonance. Thus, these 3×3=93 \times 3 = 9 lone pairs are localized.
    • Lone pair on Carbon atom: The carbon atom is bonded to three chlorine atoms and has one lone pair. The carbon atom is sp3sp^3 hybridized (3 bonds + 1 lone pair = 4 electron domains). There are no adjacent pi systems or empty orbitals on carbon that would allow for resonance or delocalization of this lone pair. Therefore, this lone pair is also localized on the carbon atom.

  4. Count the total number of localized lone pairs:

    • Localized lone pairs on chlorine atoms = 9
    • Localized lone pairs on the carbon atom = 1
    • Total number of localized lone pairs = 9+1=109 + 1 = 10.

The CCl3−CCl_3^- ion does not exhibit resonance because there is no pi system available for delocalization. All the lone pairs are confined to their respective atoms.