Trigonal planar geometry in AP Chemistry

Trigonal planar geometry is the VSEPR shape of a central atom with three bonding groups and zero lone pairs, giving a flat arrangement with 120° bond angles and sp2 hybridization (examples: BF₃, AlCl₃, NO₃⁻, and SO₃).

What is trigonal planar geometry?

Trigonal planar geometry is what you get when a central atom has exactly three electron domains and all three are bonds. VSEPR theory (EK 2.7.A.1) says electron pairs repel each other through Coulombic repulsion, so three domains spread out as far as possible. In a flat plane, the farthest apart they can get is 120° from each other. That's the whole shape: three atoms at the corners of a triangle, central atom in the middle, everything in one plane.

The critical condition is zero lone pairs on the central atom. Classic examples are BF₃ (boron only needs 6 electrons), AlCl₃ in the gas phase, formaldehyde (CH₂O), and resonance ions like NO₃⁻ and CO₃²⁻. The central atom in a trigonal planar molecule is sp2 hybridized, since three electron domains need three hybrid orbitals. If the three outer atoms are identical, the bond dipoles cancel and the molecule is nonpolar even though the individual bonds are polar.

Why trigonal planar geometry matters in AP® Chemistry

Trigonal planar is one of the named molecular geometries you're explicitly required to know under EK 2.7.A.2 in Topic 2.7 (VSEPR and Bond Hybridization), Unit 2: Compound Structure and Properties. Learning objective 2.7.A asks you to connect Lewis diagrams, VSEPR, bond order, and bond polarity to explain a molecule's structure and electron properties. Trigonal planar is the geometry where all of those threads meet. It's tied to sp2 hybridization, it's the shape of major resonance species like NO₃⁻ (where you also reason about fractional bond order), and it's the textbook setup for symmetry-cancels-dipoles polarity arguments. If you can fully explain why BF₃ is trigonal planar, nonpolar, and sp2, you've basically demonstrated the entire skill 2.7.A is testing.

How trigonal planar geometry connects across the course

sp2 Hybridization (Unit 2)

Trigonal planar geometry and sp2 hybridization are two descriptions of the same situation. Three electron domains means three hybrid orbitals, which means one s and two p orbitals mix. If you identify one, you've identified the other, and the leftover unhybridized p orbital is where pi bonds and resonance live.

Lone Pair (Unit 2)

Swap one of the three bonds for a lone pair and the geometry collapses from trigonal planar (120°, flat) to trigonal pyramidal (about 107°, like NH₃). Lone pairs repel more strongly than bonds, so they squeeze bond angles and break the flat shape. This one swap is the most common trap on geometry questions.

Molecular Polarity (Unit 2)

Trigonal planar is a go-to example of symmetry canceling polarity. BF₃ has three very polar B-F bonds, but the three dipoles point 120° apart and cancel exactly, so the molecule has no net dipole moment. Replace one F with a different atom and the cancellation breaks.

Linear Geometry (Unit 2)

Linear (2 domains, 180°), trigonal planar (3 domains, 120°), and tetrahedral (4 domains, 109.5°) form the basic VSEPR ladder. Bond angle questions often make you rank molecules across these geometries, so knowing that fewer domains means bigger angles gets you the answer fast.

Is trigonal planar geometry on the AP® Chemistry exam?

Multiple-choice questions usually give you a formula or Lewis structure and ask for the geometry, the hybridization, or the bond angle, often all in one stem. A favorite trap is the AX₃ molecule with one lone pair on the central atom (like NH₃), which is trigonal pyramidal and sp3, not trigonal planar. Ranking questions ask which molecule has the largest bond angle, and a true trigonal planar species (120°) beats anything tetrahedral or pyramidal. The nitrate ion shows up to combine geometry with resonance and bond order, since all three N-O bonds are identical with a bond order of 4/3 in a trigonal planar frame. On the free-response side, the 2023 exam built an FRQ around gaseous AlCl₃, where drawing the Lewis diagram and justifying the trigonal planar shape using VSEPR reasoning (three bonding domains, no lone pairs, electrons minimize repulsion at 120°) is exactly the move the rubric rewards. Always justify with repulsion logic, not just the shape's name.

Trigonal planar geometry vs trigonal pyramidal geometry

Both have a central atom bonded to three other atoms, which is why they get mixed up. The difference is the lone pair. Trigonal planar has zero lone pairs on the central atom (BF₃: flat, 120°, sp2). Trigonal pyramidal has one lone pair (NH₃: a 3D pyramid, roughly 107°, sp3). The lone pair pushes the three bonds downward and out of the plane. Quick check: count electron domains on the central atom, not just bonded atoms. Three domains total is planar; four domains with one lone pair is pyramidal.

Key things to remember about trigonal planar geometry

• Trigonal planar geometry occurs when a central atom has three bonding domains and zero lone pairs, producing a flat shape with 120° bond angles.

• A trigonal planar central atom is always sp2 hybridized, and its unhybridized p orbital is what allows pi bonding and resonance in species like NO₃⁻.

• If the three outer atoms are identical, the bond dipoles cancel and the molecule is nonpolar overall, which is why BF₃ has polar bonds but no net dipole moment.

• VSEPR justifications must mention Coulombic repulsion: electron domains spread to 120° because that arrangement minimizes electron-pair repulsion (EK 2.7.A.1).

• Adding a lone pair changes everything: an AX₃ molecule with one lone pair (like NH₃) is trigonal pyramidal and sp3, not trigonal planar.

• The nitrate ion (NO₃⁻) is the classic combo question, pairing trigonal planar geometry with resonance and a fractional bond order of 4/3.

Frequently asked questions about trigonal planar geometry

What is trigonal planar geometry in AP Chem?

It's the VSEPR molecular geometry for a central atom with three bonding groups and no lone pairs. The three atoms sit 120° apart in a flat plane, the central atom is sp2 hybridized, and examples include BF₃, gaseous AlCl₃, and NO₃⁻.

Is NH₃ trigonal planar?

No. Nitrogen in NH₃ has three bonds plus one lone pair, so it has four electron domains. That makes it trigonal pyramidal with sp3 hybridization and bond angles near 107°, not 120°. This is the single most common wrong answer on geometry questions.

What's the difference between trigonal planar and trigonal pyramidal?

Trigonal planar has zero lone pairs on the central atom (flat, 120°, sp2, like BF₃). Trigonal pyramidal has one lone pair (3D pyramid, about 107°, sp3, like NH₃). Count electron domains, not just bonded atoms, to tell them apart.

What hybridization goes with trigonal planar geometry?

sp2. Three electron domains require three hybrid orbitals, made by mixing one s orbital with two p orbitals. The leftover unhybridized p orbital can form a pi bond, which is why trigonal planar atoms show up in double bonds and resonance structures.

Are trigonal planar molecules always nonpolar?

No, only when all three outer atoms are identical, like in BF₃ or SO₃, where the three bond dipoles cancel by symmetry. If one outer atom is different (like in CH₂O, formaldehyde), the dipoles don't cancel and the molecule is polar.