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Trigonal Planar vs. Trigonal Pyramidal: What's the Difference?

Edited by Aimie Carlson || By Janet White || Published on March 1, 2024
Trigonal planar molecules have a 120° angle flat shape; trigonal pyramidal structures have a 3D pyramid shape with a lone pair at the apex.

Key Differences

Trigonal planar and trigonal pyramidal are terms used to describe the geometric shape of molecules based on their atomic arrangements and electron pair geometry. A trigonal planar molecule has three atoms bonded to a central atom, with all atoms in the same plane, resulting in a flat, triangular shape. This configuration leads to bond angles of approximately 120 degrees. In contrast, a trigonal pyramidal structure also has three atoms bonded to a central atom, but it includes a lone pair of electrons at the central atom, pushing the bonded atoms into a three-dimensional pyramid shape. The presence of the lone pair decreases the bond angles to slightly less than 109.5 degrees.
The distinction between trigonal planar and trigonal pyramidal geometries arises from the electron pair repulsion theory, which predicts the shape of molecules by minimizing the repulsion between electron pairs around the central atom. In trigonal planar molecules, the electron pairs (bonding pairs) are spaced evenly around the central atom, creating a flat and symmetrical shape. For trigonal pyramidal molecules, the lone pair of electrons exerts a repulsive force, distorting the geometry into a pyramid with the central atom at the apex and creating an asymmetrical distribution of charge.
The trigonal planar geometry is characteristic of molecules with a central atom bonded to three other atoms and no lone pairs, leading to sp2 hybridization. Examples include boron trifluoride (BF3) and ethylene (C2H4). Trigonal pyramidal geometry is typical for molecules with sp3 hybridization, where the central atom has three bonded pairs and one lone pair of electrons, such as ammonia (NH3) and phosphine (PH3).
One of the key differences in physical properties between trigonal planar and trigonal pyramidal molecules is their polarity. Trigonal planar molecules are often nonpolar if all three peripheral atoms are the same due to their symmetrical shape. In contrast, trigonal pyramidal molecules are usually polar because of the asymmetry introduced by the lone pair, leading to a net dipole moment.
Understanding the differences between trigonal planar and trigonal pyramidal geometries is crucial for predicting the behavior of molecules in chemical reactions, their interactions, and their physical properties. It also aids in the study of molecular biology, materials science, and various applications in chemistry and engineering, reflecting the diverse ways these molecular shapes influence the world at a microscopic level.

Comparison Chart


Flat, triangular shape
Three-dimensional pyramid shape

Bond Angles

Approximately 120°
Slightly less than 109.5°

Lone Pairs on Central Atom





Often nonpolar (if peripheral atoms are identical)
Usually polar due to asymmetry

Trigonal Planar and Trigonal Pyramidal Definitions

Trigonal Planar

Often results in nonpolar molecules when all peripheral atoms are the same.
The trigonal planar shape of BCl3 makes it nonpolar despite its polar bonds.

Trigonal Pyramidal

Involves sp3 hybridization of the central atom.
The central atom in a trigonal pyramidal H3O+ ion is sp3 hybridized, accommodating the lone pair.

Trigonal Planar

A geometry with no lone pairs on the central atom, leading to a symmetrical shape.
The trigonal planar molecule CO32- distributes charge evenly across its structure.

Trigonal Pyramidal

A molecular shape where a central atom is bonded to three others with one lone pair, creating a pyramid.
NH3 is trigonal pyramidal, which explains its distinct physical properties.

Trigonal Planar

Characterized by 120-degree bond angles around the central atom.
The trigonal planar configuration of ethylene allows for effective pi bonding.

Trigonal Pyramidal

Features bond angles slightly less than 109.5 degrees due to lone pair repulsion.
In trigonal pyramidal PH3, the lone pair affects the molecule's shape and reactivity.

Trigonal Planar

Involves sp2 hybridization of the central atom.
Acetone's trigonal planar carbon atom contributes to its chemical reactivity.

Trigonal Pyramidal

Typically results in polar molecules due to the asymmetrical distribution of electrons.
The trigonal pyramidal shape of ClO3- contributes to its strong dipole moment.

Trigonal Planar

A molecular shape with a central atom bonded to three others in a flat, triangular layout.
BF3 is trigonal planar, facilitating its use in organic chemistry reactions.

Trigonal Pyramidal

The presence of a lone pair on the central atom causes a three-dimensional structure.
The trigonal pyramidal geometry of NH3 leads to its polarity and hydrogen bonding capability.


Why are trigonal planar molecules often nonpolar?

Their symmetrical shape distributes charge evenly if peripheral atoms are identical.

Can a molecule transition between trigonal planar and trigonal pyramidal?

Only through reactions that change the number of bonds or lone pairs on the central atom.

What makes trigonal pyramidal molecules polar?

The lone pair creates an asymmetrical electron distribution, leading to polarity.

What characterizes a trigonal planar molecule?

A flat shape with 120° bond angles and no lone pairs on the central atom.

Can trigonal planar and trigonal pyramidal shapes interconvert?

No, they are defined by specific electron pair arrangements and cannot interconvert without a chemical reaction altering their structure.

How does hybridization relate to trigonal planar and pyramidal shapes?

Trigonal planar involves sp2 hybridization, while trigonal pyramidal involves sp3.

How do electron pairs determine molecular shape?

Electron pair repulsion minimizes their mutual repulsion, determining the molecule's shape.

How does trigonal pyramidal geometry differ from trigonal planar?

It has a lone pair on the central atom, creating a 3D pyramid shape.

Can trigonal planar molecules be ions?

Yes, ions can adopt this geometry if they meet the electron arrangement criteria.

Why is understanding these geometries important in chemistry?

They influence molecular properties, reactivity, and interactions.

What role do trigonal pyramidal molecules play in biology?

They're key in many biochemical processes due to their reactivity and polarity.

What impact does lone pair repulsion have on trigonal pyramidal shapes?

It reduces the bond angles to slightly less than 109.5°.

What types of molecules typically exhibit trigonal planar geometry?

Molecules with a central atom bonded to three others and no lone pairs.

Are trigonal pyramidal geometries found in large molecules?

Yes, especially in organic and inorganic compounds where central atoms have lone pairs.

How does molecular geometry affect boiling and melting points?

Polarity and molecular shape influence intermolecular forces, affecting these properties.

Can hybridization change in reactions involving these molecules?

Yes, chemical reactions can alter hybridization, affecting molecular geometry.

Can a molecule with trigonal planar geometry act as a ligand?

Yes, especially if it can donate electron pairs to form coordinate bonds.

How does the environment affect these molecular geometries?

External factors like solvent and temperature can influence, but not change, their intrinsic shapes.

What software can predict or visualize these geometries?

Computational chemistry programs like Gaussian or ChemDraw.

What experimental methods can determine these geometries?

X-ray crystallography and NMR spectroscopy are commonly used.
About Author
Written by
Janet White
Janet White has been an esteemed writer and blogger for Difference Wiki. Holding a Master's degree in Science and Medical Journalism from the prestigious Boston University, she has consistently demonstrated her expertise and passion for her field. When she's not immersed in her work, Janet relishes her time exercising, delving into a good book, and cherishing moments with friends and family.
Edited by
Aimie Carlson
Aimie Carlson, holding a master's degree in English literature, is a fervent English language enthusiast. She lends her writing talents to Difference Wiki, a prominent website that specializes in comparisons, offering readers insightful analyses that both captivate and inform.

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