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Chemical Bonding and Molecular Structure question

2022 · 29 Jun · Shift 2 · Q2
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Chemical Bonding and Molecular Structure question

2022 · 29 Jun · Shift 2 · Q2

JEE MainChemistryChemical Bonding and Molecular StructureMCQ+4 / −1
Consider the species CH4CH_4CH4​, NH 4+_4^ +4+​ and BH 4−_4^ -4−​. Choose the correct option with respect to the these species.
  1. A
    They are isoelectronic and only two have tetrahedral structures.
  2. B
    They are isoelectronic and all have tetrahedral structures.
  3. C
    Only two are isoelectronic and all have tetrahedral structures.
  4. D
    Only two are isoelectronic and only two have tetrahedral structures.
View written solutionFree

Correct answer: B

  1. Check whether the species are isoelectronic

Isoelectronic species have the same total number of electrons.

  • For CH4CH_4CH4​: Total electrons=6+4(1)=10\text{Total electrons} = 6 + 4(1) = 10Total electrons=6+4(1)=10

  • For NH4+NH_4^+NH4+​: Total electrons=7+4(1)−1=10\text{Total electrons} = 7 + 4(1) - 1 = 10Total electrons=7+4(1)−1=10 (subtract 1 because of the +1+1+1 charge)

  • For BH4−BH_4^-BH4−​: Total electrons=5+4(1)+1=10\text{Total electrons} = 5 + 4(1) + 1 = 10Total electrons=5+4(1)+1=10 (add 1 because of the −1-1−1 charge)

Thus, all three species have 101010 electrons, so they are isoelectronic.

  1. Check their shapes

Now determine the geometry around the central atom in each case.

  • CH4CH_4CH4​: Carbon forms four bond pairs and has no lone pair. ⇒tetrahedral\Rightarrow \text{tetrahedral}⇒tetrahedral

  • NH4+NH_4^+NH4+​: Nitrogen forms four bond pairs and has no lone pair (the lone pair of NH3NH_3NH3​ is used to bind H+H^+H+). ⇒tetrahedral\Rightarrow \text{tetrahedral}⇒tetrahedral

  • BH4−BH_4^-BH4−​: Boron forms four bond pairs and has no lone pair. ⇒tetrahedral\Rightarrow \text{tetrahedral}⇒tetrahedral

Thus, all three have tetrahedral structures.

  1. Evaluate the options

  • A: They are isoelectronic and only two have tetrahedral structures.
    False, because all three are tetrahedral.

  • B: They are isoelectronic and all have tetrahedral structures.
    True.

  • C: Only two are isoelectronic and all have tetrahedral structures.
    False, because all three are isoelectronic.

  • D: Only two are isoelectronic and only two have tetrahedral structures.
    False.

  1. Final answer

The correct option is: B\boxed{\text{B}}B​

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