Boron configuration diagram
One of the three boron electrons is unpaired in its ground state. The atomic s- and p-orbitals in boron's outer shell mix to form three equivalent hybrid orbitals. These particular orbitals are called sp2 hybrids, meaning that this set of orbitals derives from one s- orbital and two p-orbitals of the free atom.
Source
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"Hybrid orbitals - 1."
http://www.chem1.com/acad/webtext/chembond/cb06.html#SEC1
Steve Lower
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