Answer
Lewis acid: $BF_3$
Lewis base: $F^-$
Work Step by Step
1. Determine if each compound can act as a Lewis acid, base, or both.
- $BF_3$: The boron has, naturally, 3 electrons in its valence shell, and it is making 3 covalent bonds with the fluorines, giving us a total of 6 electrons. Since this element is from the second row, it is capable of holding 8 electrons, which means that it can accept 2 more, making this molecule a Lewis acid.
- $F^-$: Can act a Lewis base, because it has lone pairs that can be donated.
2. So, we just have one option:
$BF_3$ as an acid, and $F^-$ as a base.
