How to tell when a molecule is an acid or base

[Michele Shi 1K]

How can I distinguish a molecule as an acid or base? I know that acids accept electrons and bases donate electrons. But by just looking at the molecular formula (BF3 for example), how do I know that Boron will not have an octet because it will accept another lone pair? Thank!

[Zane Mills 1E]

Just like you stated, Lewis acids (not to be confused w bronsted acids) accept electrons and lewis bases donate them. An easy way to determine this is to look at formal charges on molecules. Positive formal charges indicate electron deficiency and thus a lewis acid, while negative formal charges indicate excess electrons (lewis base). In this case of boron trifluoride, boron has an open valence shell that can be occupied by the excess electrons on fluoride, thus BF3 acts as a lewis acid while the fluorine acts as a base by donating it's electrons.

[Julie Steklof 1A]

You could draw Lewis structures and see if a molecule has an element with an open octet (for example Boron in BF3). This tells you that the molecule is a Lewis acid. Similarly, if you draw a Lewis structure and see that an element has an unshared pair of electrons to be able to donate. This would be a Lewis base