Boron trichloride lewis structure (BCl3)
The boron trichloride lewis structure places boron at the center with three single B–Cl bonds and no lone pairs on boron. Each chlorine carries three lone pairs. Boron has only six electrons in its valence shell in this structure, representing the incomplete-octet exception typical of Group 3A (13) central atoms.
Valence-electron accounting
The total number of valence electrons in BCl3 follows directly from periodic-group valences: \[ 3 + 3 \times 7 = 24 \] Three single bonds contain \(3 \times 2 = 6\) bonding electrons, leaving \(24 - 6 = 18\) electrons as lone pairs. Those 18 electrons correspond to nine lone pairs distributed as three lone pairs on each chlorine.
Formal charges and the stability of the incomplete octet
The formal charge expression provides a compact consistency check: \[ \text{FC} = \text{valence} - \left(\text{nonbonding} + \frac{\text{bonding}}{2}\right) \] For boron in BCl3, nonbonding electrons are 0 and bonding electrons are 6, giving \( \text{FC(B)} = 3 - (0 + 6/2) = 0 \). For each chlorine, nonbonding electrons are 6 and bonding electrons are 2, giving \( \text{FC(Cl)} = 7 - (6 + 2/2) = 0 \). A structure with all formal charges at zero is typically the preferred introductory Lewis representation.
The incomplete octet in BCl3 is not a mistake in electron counting. It reflects boron’s three valence electrons and the fact that stable main-group compounds can exist with fewer than eight electrons at a Group 3A central atom.
Geometry and bonding description
Three electron domains around boron correspond to trigonal planar geometry, consistent with an sp2-like bonding picture and bond angles near 120°. Each B–Cl bond is polar, but the trigonal planar symmetry makes the molecular dipole moment approximately zero when the molecule is idealized as planar and symmetric.
Lewis acidity and electron-pair acceptance
Electron deficiency at boron is chemically consequential: BCl3 behaves as a Lewis acid because an empty valence orbital can accept an electron pair from a donor. Adduct formation (for example, with amines or ethers) follows naturally from the incomplete-octet Lewis picture without changing the electron count on chlorine atoms in the starting molecule.
Common pitfalls
| Pitfall | Chemistry correction |
|---|---|
| Forcing an octet on boron by drawing B=Cl double bonds as the default | The simplest Lewis structure keeps three single bonds and zero formal charges; introducing B=Cl bonding changes electron distribution and is not the standard baseline for Group 3A incomplete-octet examples. |
| Placing lone pairs on boron in BCl3 | Boron contributes three valence electrons and forms three bonding pairs; no electrons remain on boron as lone pairs in the neutral molecule. |
| Assigning a permanent molecular dipole to a symmetric trigonal planar structure | Individual bond dipoles exist, but the vector sum cancels for an ideal trigonal planar arrangement with identical substituents. |