# Why is the 8N (octet) rule not applicable in the case of boron triflouride?

In my chemistry class we learned the so-called "8N rule" (very similar to the Octet rule) which helps one determine the number of bonding electron pairs and thus the number of bonds in certain molecules:

$$\text{No: of bonding electrons} = 2\cdot\text{No: of hydrogen atoms} + 8\cdot \text{No: of other atoms} - \text{Total no: of valence electrons}$$

This tends to work for most molecules I've tried. The lecture notes don't really mention any exceptions or limitations. Therefore, I assumed that it would work for a wide variety of molecules, especially ones that involve p-block atoms only.

However, the rule doesn't work with $$\ce{BF3}$$. Here I get $$2\cdot0 + 8\cdot 4 - (3+3\cdot7) = 8$$ bonding electrons which should translate to $$8/2 = 4$$ bonds (or one of the $$\ce{B-F}$$ bonds is a double bond). But this is apparently not the case - there are three single bonds or 6 bonding electrons. So where is my mistake?

I should say that this is not the first time I've encountered a rather simple molecule where the rule doesn't apply which is what gives confidence that it's not because of a mistake I made but because of a limitation of this rule that I don't know.

• Octect rule is more than 100 years old. It does not apply to many compounds. People have come up with better explanations and bonding theories.
– ACR
Commented Oct 4, 2022 at 16:50
• Then again, the bonds in BF3 are at least somewhat double. Commented Oct 4, 2022 at 17:05
• In that case, is there a better and yet as handy alternative for calculating the number of bonds or the bond order? Commented Oct 4, 2022 at 18:31
• The octet rule is a somewhat surprising model. It holds quite well for molecules as long as you are reading the 8 as an upper limit. Since it had never been formalised, this role of thumb is very open for interpretation. Commented Dec 28, 2022 at 13:48

• The octet rule has a lot of exceptions. Examples : $\ce{B2H6}$, $\ce{NO}$, $\ce{NO2}$, $\ce{SF6}$, $\ce{H2SiF6}$, plus the majority of compounds containing transition metals. Commented Oct 4, 2022 at 19:21