Boron trifluoride (BF₃) is an interesting molecule to study because of its molecular geometry, which can be well explained by Valence Shell Electron Pair Repulsion (VSEPR) theory.
Boron trifluoride (BF3) has a trigonal planar molecular geometry. Here’s a breakdown of its structure:
- Central Atom: Boron (B) which has three valence electrons.
- Surrounding Atoms: Three fluorine (F) atoms, each contributing one electron to a bond with boron.
Boron trifluoride (BF3) consists of a boron atom that is single-bonded to three fluorine atoms. Each fluorine atom starts with seven valence electrons and uses only one of them to bond with boron, leaving it with three lone pairs to complete its octet. In contrast, the boron atom has no lone pairs because it started out with three valence electrons and used all of them to bond with fluorine. See Figure 1 for the Lewis structure of boron trifluoride.
Key Features of BF3 Geometry
- Electron Pair Arrangement: Boron, in its ground state, has an electron configuration of 1s²2s²2p¹. When it forms covalent bonds with fluorine, it uses three sp² hybridized orbitals to bond with the three fluorine atoms.
- Shape: Since there are three bonding pairs and no lone pairs on the boron atom, the molecule adopts a trigonal planar shape where the bond angles are ideally 120 degrees.
- Bonding: Each B-F bond is formed by the overlap of one sp² orbital from boron with a p orbital from fluorine. The bonds are sigma (σ) bonds.
- Molecular Polarity: Although each B-F bond is polar due to the difference in electronegativity between boron and fluorine, the molecule as a whole is nonpolar because of its symmetrical shape. The dipole moments of the three B-F bonds cancel each other out due to the planar trigonal geometry.
- Electron Deficiency: Boron in BF3 has only six electrons in its outer shell after bonding (an “incomplete octet”), making it electron-deficient. This makes BF3 a Lewis acid, meaning it can accept an electron pair to complete the octet.
Additional Notes
- BF₃ is often used as a Lewis acid in chemical reactions because it can accept a pair of electrons to form a coordinate bond, completing boron’s octet. This behavior arises because boron in BF₃ has an incomplete octet, having only 6 electrons around it after bonding with the three fluorines.