Multiple choice

The allotrope of boron containing unit cell of twelve boron atoms is

  1. amorphous boron

  2. β-rhombohedral boron

  3. α-rhombohedral boron

  4. γ-boron
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

α-rhombohedral boron has a unit cell of twelve boron atoms. The structure consists of B12 icosahedra in which each boron atom has five nearest neighbors within the icosahedron. If the bonding were the conventional covalent type then each boron would have donated 5 electrons.

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