BF3 is a classic incomplete-octet exception. Boron forms three single bonds to fluorine and is surrounded by only six electrons, while each fluorine has a complete octet.
Three B–F single bonds; three lone pairs on each fluorine
Total valence electrons24
Central atomBoron
Molecular geometryTrigonal planar
Bond angle120°
BF3 Lewis structure overview
Boron contributes three valence electrons and three fluorines contribute twenty-one, giving twenty-four total. Three B–F bonds use six electrons. The remaining eighteen electrons become three lone pairs on each fluorine. Boron has six electrons around it, an accepted exception for electron-deficient group 13 compounds.
How to draw the BF3 Lewis structure
Count 24 valence electrons.
Use 3 from boron and 7 from each fluorine.
Place boron in the center.
Boron is less electronegative than fluorine and can connect to all three terminal atoms.
Draw three B–F single bonds.
The bonds use 6 electrons.
Complete fluorine octets.
Give each fluorine three lone pairs, using the remaining 18 electrons.
Accept boron's incomplete octet.
Creating B=F double bonds would introduce unfavorable formal charges in the common introductory structure.
Formal charges and octet exception
All atoms have formal charge zero in the three-single-bond structure. Boron is electron deficient, which helps explain why BF3 acts as a Lewis acid and accepts an electron pair from a donor.
Geometry and polarity
Boron has three bonding domains and no lone pairs, producing trigonal planar geometry. The B–F bonds are polar, but the symmetric 120° arrangement cancels their dipoles, so isolated BF3 is nonpolar overall.
Common mistakes
Forcing boron to have eight electrons by drawing three B=F double bonds.
Leaving lone pairs off fluorine.
Calling BF3 trigonal pyramidal; boron has no lone pair.
Assuming every second-period central atom must always satisfy an octet.
Frequently asked questions
Why is BF3 stable with only six electrons around boron?
Boron commonly forms electron-deficient compounds. BF3 can accept an electron pair when it reacts with a Lewis base.
Does BF3 have resonance?
Some advanced bonding descriptions include π donation from fluorine, but the standard introductory Lewis structure uses three single bonds and an incomplete boron octet.