BONDING CAPACITY OF BORON

Although the boron atom (with electron configuration Is2 2s2 2p ) has three valence electrons, only one of them is unpaired in the ground state. 

To increase the number of unpaired electrons, one electron is promoted from the 2s orbital to a 2p orbital. Then the 2s and two 2p orbitals mix to form three identical sp2 hybrid orbitals. 

As a result of hybridization, boron can form three bonds and so the BH3 molecule is formed with hydrogen. 

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