What is diffrence between 4 terminal hydrogen atoms and bridge H atoms?
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By the tag words you appear to be talking about diborane. Diborane has a formula of B2H6. It is basically a dimer of the BH3 monomer. Boron has three valence electrons, so it can form three regular covalent single bonds with three hydrogen atoms. But as the dimer B2H6, things are different. Four of the hydrogen atoms are terminal hydrogens with standard covalent single bonds. But the remaining two hydrogens are called bridge hydrogens. They act as bridges between the two boron atoms. But they do have a different type of bonding. This bridge bond is a 3-center-2-electron bond. There are two electrons being shared between three atoms (H-B-H). The resulting bond is slightly longer than a standard single bond, thus reflecting the relative weakness of the bridge bond compared to the terminal bond. Certain elements including boron are capable of forming this special type of bond.
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