What is the Lewis dot structure of ##BH_3##? How many lone pair electrons are in this molecule? How many bonding pairs of electrons are in this molecule? How many lone pair electrons are at the central atom?

Well, there are 6 electrons to distribute in ##BH_3##, however, ##BH_3## does not follow the pattern of ##"2-centre, 2 electron"## bonds.

Boron has 3 valence electrons, and hydrogen has the 1; thus there are 4 valence electrons. The actual structure of borane is as diborane ##B_2H_6##, i.e. ##{H_2B}_2(mu_2-H)_2##, in which there are ##"3-centre, 2 electron"## bonds, bridging hydrogens which bind to 2 boron centres. I would suggest that you get your text, and read in detail how such a bonding scheme operates.

By contrast, in ethane, ##C_2H_6##, there are enough electrons to form ##7xx"2-centre, 2 electron"## bonds, i.e. the ##C-C## linkage, and ##6xx"C-H"## bonds.