Lewis Structure of Group 3A Central Atoms Incomplete Octet

General Chemistry • Chemical Bonds

Written by STEM Calculators Team Published December 1, 2025 Updated April 21, 2026

Group 13 Central Atoms – Incomplete Octet

Explore classic electron-deficient molecules such as BF₃, BCl₃, BH₃, and AlCl₃. This calculator counts valence electrons, shows why the central Group 13 atom has only 6 electrons, and visualizes why these species behave as Lewis acids.

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Every example here uses a Group 13 central atom with three single bonds, no lone pairs on the central atom, and a 6-electron valence shell instead of a full octet.

Teaching focus

The chemistry stays the same in every mode. This changes the annotations so the calculator can emphasize octet deficiency or lone-pair acceptance.

Core rule for this topic: total valence electrons = contribution from the Group 13 atom + 3 × contribution from the surrounding atoms. The central atom forms three σ bonds, so it is surrounded by 6 electrons, leaving it 2 electrons short of a complete octet.

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Trigonal planar Lewis diagram

Electron budget and octet map

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#	Molecule	Total valence	Central electrons	Octet deficiency	Total lone pairs	Lewis acid

Click a row to inspect that molecule in the diagrams and the step-by-step explanation.

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Group 13 central atoms – incomplete octet (BF₃, BCl₃, BH₃, AlCl₃…)

Atoms in Group 13 (B, Al, …) have three valence electrons \((\mathrm{ns}^2\mathrm{np}^1)\). When they form three single bonds to surrounding atoms, the central atom is surrounded by only three bonding pairs (6 electrons), so the octet rule is not satisfied. These are classic electron-deficient molecules.

For example, in \(\mathrm{BF_3}\):

\begin{aligned} N_{\text{valence}} &= 3(\mathrm{B}) + 3\cdot 7(\mathrm{F}) = 24 \\ N_{\text{bonding}} &= 3\cdot 2 = 6 \quad (\text{three B{-}F bonds}) \\ N_{\text{nonbonding}} &= 24 - 6 = 18 \quad (\text{lone pairs on fluorine}) \\ \text{Electrons around B} &= 3 \text{ bonding pairs } = 6~e^- \quad \Rightarrow \text{incomplete octet.} \end{aligned}

With three electron domains and no lone pairs on the central atom, VSEPR predicts an \(\text{AX}_3\) trigonal planar geometry and bond angles close to \(120^\circ\). This applies to BF₃, BCl₃, BBr₃, BH₃ and AlCl₃ (considering an individual Al center).

Species	Central atom	Electrons around central atom	VSEPR code	Geometry
BF₃	B	3 bonds = 6 e⁻	AX₃	Trigonal planar
BCl₃ / BBr₃	B	3 bonds = 6 e⁻	AX₃	Trigonal planar
BH₃ (local picture)	B	3 bonds = 6 e⁻	AX₃	Trigonal planar
AlCl₃ (single Al center)	Al	3 bonds = 6 e⁻	AX₃	Trigonal planar

Because the central atom is electron-poor, these molecules are strong Lewis acids: they readily accept an electron pair from a Lewis base (for example, \(\mathrm{BF_3 + NH_3 \rightarrow F_3B{-}NH_3}\)). In the solid state or in some phases, BH₃ and AlCl₃ often dimerize (B₂H₆, Al₂Cl₆), but the local Lewis environment at each B/Al center is still three σ bonds, no lone pairs, and 6 electrons.

Frequently Asked Questions

What does an incomplete octet mean in a Lewis structure?

An incomplete octet means the central atom has fewer than 8 valence electrons around it in the final Lewis structure. For Group 13 central atoms with three single bonds, the central atom is surrounded by 6 electrons.

Why do Group 13 molecules like BF3 have only 6 electrons around boron?

Boron has only 3 valence electrons, so forming three single bonds creates three bonding pairs around B. That gives 6 electrons around the boron center, which is an acceptable electron-deficient exception to the octet rule.

Why are these Group 13 central-atom structures trigonal planar?

The central atom has three electron domains (three sigma bonds) and no lone pairs. VSEPR describes this as AX3, which corresponds to a trigonal planar arrangement.

How is an incomplete octet related to Lewis-acid behavior?

A central atom with only 6 electrons is electron-poor and can accept an electron pair from a donor. This is why molecules like BF3 and AlCl3 are commonly classified as Lewis acids.

Do BH3 and AlCl3 always exist as the simple molecules shown in a Lewis diagram?

In reality, BH3 often forms B2H6 and AlCl3 can form dimers or polymers depending on conditions. The calculator focuses on the local Lewis description at the B or Al center, which remains electron-deficient and trigonal planar in that simplified picture.