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GATE 2021 CY – Question 39

Inorganic Chemistry · Solids: Crystal systems and lattices, Miller planes, crystal packing, crystal defects, Bragg’s law, ionic crystals, structure of AX, AX2, ABX3 type compounds, spinels, band theory, metals and semiconductors, Zeolites and their applications. · 2 marks · Multiple choice

The metal borides that contain isolated boron atoms are:

  1. Tc7B3 and Re7B3
  2. Cr5B3 and V3B2
  3. Ti4B4 and V3B4
  4. TiB and HfB

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Correct answer: (A) Tc7B3 and Re7B3

Explanation

Metal-rich borides can separate boron atoms so that there is no extended B–B network. **Tc7B3 and Re7B3, A**, belong to the isolated-boron structural class. The other listed compositions contain bonded boron units or chains rather than isolated boron throughout. The distinction concerns B–B connectivity, not merely whether the formula is metal-rich.