Trigonal pyramidal geometry is the molecular shape that forms when a central atom has three bonding pairs and one lone pair (AX₃E), giving an sp³-hybridized atom with bond angles slightly less than 109.5° (about 107°), as in ammonia (NH₃).
Trigonal pyramidal is the shape you get when a central atom is bonded to three other atoms and also holds one lone pair. Count the electron domains and you get four total, so the electron-pair geometry is tetrahedral. But molecular geometry only describes where the atoms are, and the lone pair sits invisibly at one corner. The three bonded atoms form the base of a pyramid with the central atom at the top. The classic example is ammonia, NH₃.
VSEPR theory (2.7.A.1) explains the shape through Coulombic repulsion. Electron pairs push each other as far apart as possible, and a lone pair pushes harder than a bonding pair because it's held closer to the central atom. That extra squeeze compresses the bond angles from the perfect tetrahedral 109.5° down to roughly 107°. The central atom is sp³ hybridized, since hybridization follows the four electron domains, not the three visible bonds.
This term lives in Topic 2.7 (VSEPR and Bond Hybridization) in Unit 2, and it's named explicitly in essential knowledge 2.7.A.2 as one of the geometries you have to predict from a Lewis diagram. Learning objective 2.7.A asks you to connect Lewis structures, VSEPR, and bond polarity to explain a molecule's structural and electronic properties. Trigonal pyramidal is the single best test of whether you actually understand that chain of reasoning, because it forces you to separate electron-pair geometry (tetrahedral) from molecular geometry (trigonal pyramidal). It also matters for polarity. The three bond dipoles in a trigonal pyramidal molecule don't cancel, so the molecule has a net dipole moment, which feeds directly into intermolecular force and boiling point arguments later in the course.
Keep studying AP® Chemistry Unit 2
Lone pair (Unit 2)
The lone pair is the whole reason this geometry exists. Remove it and AX₃ becomes flat trigonal planar; keep it and the lone pair shoves the three bonds downward into a pyramid and squeezes the angles to about 107°.
Molecular polarity and dipole moment (Unit 2)
Trigonal pyramidal molecules with polar bonds are always polar. The asymmetry from the lone pair means the three bond dipoles can't cancel, so a molecule like NH₃ has a permanent dipole moment, unlike symmetric shapes where dipoles cancel out.
Intermolecular forces and boiling points (Unit 3)
Geometry is the bridge from structure to physical properties. Because trigonal pyramidal NH₃ is polar (and N-H bonds allow hydrogen bonding), it has a much higher boiling point than nonpolar molecules of similar size. Exam questions love this structure-to-property chain.
sp2 hybridization vs sp³ (Unit 2)
A trigonal planar central atom (three domains, no lone pair) is sp², while a trigonal pyramidal central atom is sp³ because the lone pair counts as a fourth domain. Hybridization tracks electron domains, not just bonds.
Multiple-choice questions typically hand you a description like "a central atom with three bonding pairs and one lone pair" or an AX₃E formula and ask for the geometry, the hybridization, or both together (sp³ and trigonal pyramidal must be paired correctly). A common trap answer splits them up, offering sp² with trigonal pyramidal. Questions also test the distinction between electron-pair geometry (tetrahedral) and molecular geometry (trigonal pyramidal) for the same molecule. On FRQs, you're usually asked to draw a Lewis structure, name or sketch the geometry, estimate the bond angle, and then use the shape to justify whether the molecule is polar. Always mention the lone pair explicitly in your reasoning. Saying "the lone pair makes the shape asymmetric, so bond dipoles don't cancel" is the kind of sentence that earns the point.
Both have three atoms attached to the central atom, but trigonal planar (AX₃, like BF₃) has no lone pair, so it's flat with 120° angles and an sp² center. Trigonal pyramidal (AX₃E, like NH₃) has one lone pair, so it's a 3D pyramid with ~107° angles and an sp³ center. The quick check is to count electron domains, not just bonded atoms. Three domains means planar; four domains with one lone pair means pyramidal. The polarity consequence is huge too, since planar AX₃ molecules with identical outer atoms are nonpolar while pyramidal ones are polar.
Trigonal pyramidal geometry occurs when a central atom has three bonding pairs and one lone pair, the AX₃E arrangement, with NH₃ as the classic example.
The electron-pair geometry is tetrahedral, but the molecular geometry is trigonal pyramidal because the lone pair occupies a corner without counting as part of the shape.
Bond angles are about 107°, slightly less than the tetrahedral 109.5°, because the lone pair repels bonding pairs more strongly.
The central atom is sp³ hybridized, since hybridization is determined by all four electron domains, not just the three bonds.
Trigonal pyramidal molecules with polar bonds have a net dipole moment because the asymmetric shape prevents bond dipoles from canceling.
Don't confuse it with trigonal planar, which has three bonds, zero lone pairs, 120° angles, and an sp² central atom.
It's the molecular shape formed when a central atom bonds to three atoms and holds one lone pair (AX₃E). The lone pair pushes the three bonds into a pyramid shape with bond angles around 107°, like in ammonia, NH₃.
No, but they're related. A trigonal pyramidal molecule has a tetrahedral electron-pair geometry because it has four electron domains total, but its molecular geometry is trigonal pyramidal since one of those domains is a lone pair, not an atom.
Trigonal planar (like BF₃) has three bonds and no lone pairs, making it flat with 120° angles and sp² hybridization. Trigonal pyramidal (like NH₃) has three bonds plus one lone pair, making it a 3D pyramid with ~107° angles and sp³ hybridization.
Approximately 107°. It starts from the ideal tetrahedral angle of 109.5°, but the lone pair repels the bonding pairs more strongly than they repel each other, compressing the angle slightly.
Yes, if the bonds are polar. The lone pair makes the shape asymmetric, so the three bond dipoles can't cancel and the molecule has a net dipole moment. That's why NH₃ is polar while flat, symmetric BF₃ is not.
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