How does BF4^- make tetrahedral structure. Because B have 3 outermost electrons. It may combine with three fluorine. But how is it combining with 4?

Answer 1

B has 5 electrons total and three outside electrons so it is

.B.
.

You note that the three electrons, all unpaired, leaves that fourth spot (at the top)open so one F shares the electron to the left spot, another to the right spot, another to the bottom spot, and the top spot has a F^- ION sit down there. That gives each F an octet, the top F^- is a coordinate covalent bond with an octet and the whole thing has a -1 charge.