Molecules generally have as many electrons as they have protons. Boron’s got 5 protons, and thus 5 electrons. Fluorine has 9 protons, and 9 electrons. Electrons in the inner shells don’t really get a chance to react with things, so we don’t really have to consider the innermost shell (which holds 2 electrons). So Boron has 3 electrons in its outer shell, and Fluorine has 7 electrons in its outer shell.
In general, when bonding atoms try to get to a full outer shell in the most efficient way – shedding extra electrons or picking up electrons, whichever involves moving the least number of electrons.
Each fluorine takes an electron off of Boron, so they each have 8; now they’ve got a full outer shell. Boron loses 3 electrons, so it’s down to its innermost (2 electron) shell. All the atoms now have a full outer shell – and that’s all the electrons we’ve got. There are no spare electrons on Boron to repel the Fluorine atoms.
If we were talking about [*Nitrogen* Trifluoride](https://en.wikipedia.org/wiki/Nitrogen_trifluoride), we’d have two more electrons, which would stay on the nitrogen, repelling the Fluorine atoms and resulting in a pyramid shape.
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