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How is pure crystalline boron prepared from boron halides?

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A labeled diagram of the periodic table entry for boron shows its atomic number 5, symbol B, electron configuration, atomic weight range, and classification as a solid, weakly acidic, other nonmetal with a rhombohedral crystal structure at room temperature. — boron Properties of boron.

Pure crystalline boron can be prepared by the reduction of its halides—specifically boron tribromide ( ${BBr}_{3}$ ) or boron trichloride ( ${BCl}_{3}$ )—with hydrogen gas.1

This process is performed on an electrically heated tantalum filament. The preparation of pure crystalline boron in this manner is considered difficult.1