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Animation of the bonding in a borane molecule (BH3). Initially a boron atom has three populated electron orbitals; an inner spherical 1s shell, a spherical 2s shell, and one bi-lobed 2p shell. It might be expected that only the 2p orbital would be involved in bonding, producing BH, but instead one 2s electron (red and white) rises to a vacant 2p level (dark red), and the two 2p orbitals and the 2s orbital hybridise, producing three identical sp2 orbitals, arranged in a trigonal planar (flat triangle) shape. These then bond with hydrogen atoms, with a release of energy. The hybridised BH3 is energetically favoured over BH as the energy required to raise the 2s electron to the 2p level is more than made up for by the energy released in the creation of the two extra B-H bonds that the hybridisation permits.
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