Lewis Structure of Boron Trifluoride (BF3)

Boron trifluoride contains one boron atom and three fluorine atoms. Lewis structure of boron trifluoride (BF3) is shown below and you can see each fluorine atom has made a single bond with boron atom. Boron atom is the center atom. In this tutorial, we will learn how to draw the lewis structure of BF3 with all theories.

BF3 lewis structure

BF3 lewis structure

According to the lewis structure of BF3, there are only six electrons around boron atom. Therefore, octal of boron atom is not completed. Therefore, borane BF3 is considered as a lewis acid.

Steps of drawing lewis structure of BF3

There are general guidelines to draw a lewis structure step by step and they are mentioned below. In this lesson, we use those rules to draw the BF3 lewis structure and they are explained in detail in next sections of this tutorial. If you are are beginner to lewis structure drawing, follow these sections slowly and properly to understand.

  1. Find total number of electrons of the valance shells of boron and fluorine atoms
  2. Total electrons pairs
  3. Center atom selection
  4. Put lone pairs on atoms
  5. Check the stability and minimize charges on atoms by converting lone pairs to bonds until most stable structure is obtained.

Total number of electrons of the valance shells of BF3

  • Boron is a group IIIA element. Therefore, boron has three electrons in its last shell.
  • Fluorine is a group VIIA element. Therefore, fluorine has seven electrons in its last shell.

  • Total valence electrons given by boron atom = 3 * 1 = 3

There are three hydrogen atoms in BF3 molecule, Therefore

  • Total valence electrons given by fluorine atoms = 7 * 3 = 21

  • Total valence electrons = 3 + 21 = 24

Total valence electrons pairs

Total valance electrons pairs = σ bonds + π bonds + lone pairs at valence shells

Total electron pairs are determined by dividing the number total valence electrons by two. For, BF3, total pairs of electrons are 12 (= 24/2).

Center atom and sketch of BF3

There are requirements to be the center atom. Having a high valence and being the most electropositive atom are the most important facts to be the center atom. In BF3, there are only two elements to select the center atom. So, selecting the center atom is not a hard thing for BH3.

  • Boron's maximum valence is 3. Fluorine's maximum valence is 1. From that fact, boron has the high priority to be the center atom.
  • Boron's and fluorine's electronegativity values are 2 and 4.0 respectively. Therefore boron is more electropositive than fluorine. From that case too, boron has the highest chance to be the center atom.
  • From above two facts, we can decide boron should be the center atom of BF3.
BF3 sketch

Mark lone pairs on atoms

Now, we should mark lone pairs on atoms. At the beginning, lone pairs should be marked on outside atoms ( in this case fluorine atoms).

  • There are three B-F bonds in the above sketch. Therefore, there are more 9 valence electrons pairs to mark on atoms.
  • Mark these lone pairs first o fluorine atoms. Each fluorine atom will take three lone pairs. So, nine lone pairs are spent. Then, all valence electrons pairs are finished and there are no more lone pairs to mark on boron atom.
mark lone pairs on BF3 lewis structure

Charges on atoms

In above sketch, there are no charges on atoms. Therefore, we don't need to reduce charges to obtain most stable structure. That means, we already found the lewis structure of BF3.


What is the shape of BF3?

There are only three sigma bonds around boron atom. Therefore, shape of BF3 should be Trigonal Planar.

Why fluorine atom cannot be the center atom?

Fluorine's maximum valence is one. Therefore, fluorine cannot make two more bonds around fluorine atom. Also, electronegativity of fluorine is very high. Usually, center atom should be more electropositive.