JEE Challenger
More from p-Block Elements
ChemMedAdv 2016 • Paper 1

Structural Properties of Crystalline Borax

Which of the following statements is/are correct regarding the crystalline structure of borax?

Options

A

contains a tetranuclear [B4O5(OH)4]2−[B_4O_5(OH)_4]^{2-} unit

Correct
B

all boron atoms lie within the same plane

C

equal numbers of sp2sp^2 and sp3sp^3 hybridized boron atoms

Correct
D

one terminal hydroxide group per boron atom

Correct

Topics & Concepts

p-Block Elementsp-Block Elements

Step-by-Step Solution

To determine the correct statements regarding the crystalline structure of borax, let us analyze its molecular and crystal structure in detail:

  1. Structural Formula and Anionic Unit (Statement A): Borax is conventionally represented as Na2B4O7⋅10H2O\text{Na}_2\text{B}_4\text{O}_7 \cdot 10\text{H}_2\text{O}. However, structural analysis reveals that it actually consists of sodium ions, water molecules, and discrete tetranuclear anionic units. Its structural formula is accurately written as: Na2[B4O5(OH)4]⋅8H2O\text{Na}_2[\text{B}_4\text{O}_5(\text{OH})_4] \cdot 8\text{H}_2\text{O} The basic structural unit of borax is the tetranuclear anion [B4O5(OH)4]2−[\text{B}_4\text{O}_5(\text{OH})_4]^{2-}. Thus, Statement (A) is correct.

  2. Planarity of Boron Atoms (Statement B): In the [B4O5(OH)4]2−[\text{B}_4\text{O}_5(\text{OH})_4]^{2-} ion:

    • Two boron atoms adopt a tetrahedral geometry (3D shape).
    • Two boron atoms adopt a trigonal planar geometry.

    Because of the presence of tetrahedral sp3sp^3 hybridized boron atoms bridging across different planes, the four boron atoms do not all lie in the same plane. Thus, Statement (B) is incorrect.

  3. Hybridization of Boron Atoms (Statement C): Within the [B4O5(OH)4]2−[\text{B}_4\text{O}_5(\text{OH})_4]^{2-} ion:

    • 22 boron atoms are bonded to 33 oxygen atoms each in a trigonal planar arrangement   ⟹  sp2 hybridized\implies sp^2\text{ hybridized}.
    • 22 boron atoms are bonded to 44 oxygen atoms each in a tetrahedral arrangement   ⟹  sp3 hybridized\implies sp^3\text{ hybridized}.

    Since there are 2 sp2sp^2 hybridized and 2 sp3sp^3 hybridized boron atoms, there is an equal number of both types of hybridized boron atoms. Thus, Statement (C) is correct.

  4. Terminal Hydroxide Groups (Statement D): In the [B4O5(OH)4]2−[\text{B}_4\text{O}_5(\text{OH})_4]^{2-} structure, five oxygen atoms act as bridges between boron atoms (−O−-\text{O}- linkages), while the four hydroxide (−OH-\text{OH}) groups are attached as terminal ligands to each of the four boron atoms (one terminal −OH-\text{OH} group per boron atom). Thus, Statement (D) is correct.

Conclusion: The correct statements are (A), (C), and (D).

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