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The p-Block Elements — Chemistry STD 11 Science — Question

Gujarat BoardEnglish MediumSTD 11 ScienceChemistryThe p-Block Elements3 Marks
Question
$BCl _3$ is trigonal planar while $AlCl _3$ is tetrahedral in dimeric state. Explain.
OR
$BCl _3$ exists as monomer whereas $AlCl _3$ is dimerised through halogen bridging. Give reason. Explain the structure of the dimer of $AlCl _3$ also.
✓

Answer

Both $BCl _3$ and $AlCl _3$ are electron deficient molecules having six electrons in the valence shell of their respective central atoms. To complete their octets, the central atom in each case can accept a pair of electrons from the chlorine atom of another molecule forming dimeric structures. However, because of small size of B , it cannot accommodate four big sized Cl atoms around it. Therefore, $BCl _3$ prefers to exist as a monomeric planar molecule in which B atom is $sp ^2$-hybridised.

On the other hand, Al because of its bigger size can easily accommodate four Cl atoms around it. As a result, $AlCl_3$ exists as a dimer. In this dimer, since the covalency of Al has increased to 4.
Therefore, Al is $sp^3$​​​​​​​-hybridised and the four Cl atoms are held tetrahedrally around it.

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