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Trigonal Pyramidal Molecule Geometry: Explained & Examples

Trigonal pyramidal geometry is a shape found in compounds with a central atom that has four electron domains. It has three bonding pairs and one lone pair of electrons. The bond angles in this shape are about 107 degrees.

Knowing about trigonal pyramidal geometry is important because it affects the properties of compounds. It determines how atoms and orbitals are arranged, which affects how molecules interact with each other.

Difference between Trigonal Pyramidal and trigonal Planar:

Trigonal pyramidal and trigonal planar are two different geometries that describe the arrangement of electron domains around a central atom. While they may sound similar, there are some key differences between them.

Electron Domain Arrangement

In trigonal planar, the central atom is surrounded by 3 bonding pairs of electrons in a flat, triangle shape. There are no lone pairs on the central atom. In trigonal pyramidal, there are also 3 bonding pairs of electrons but one lone pair on the central atom.

Effect on Shape and Bond Angles

The shape and bond angles of molecules can be affected by a lone pair. In trigonal planar molecules like BF3, the bond angles are all 120 degrees because there are no lone pairs. But in trigonal pyramidal molecules like NH3, the bond angle is slightly less than 109.5 degrees because of repulsion from the lone pair.

Shape Difference

Some molecules are flat and look like a triangle when you see them from above or below. This is called trigonal planar geometry. Formaldehyde (CH2O) is an example of a molecule with this shape.

On the other hand, some molecules have a shape that looks like a pyramid because there is a lone pair on top of the three bonded atoms. This is called trigonal pyramidal geometry. Ammonia (NH3) and phosphine (PH3) are examples of molecules with this shape.

Understanding these differences between trigonal pyramidal and trigonal planar geometries is crucial for understanding molecular shapes and predicting their properties.

Examples of Trigonal Pyramidal Molecules


Ammonia (NH3)

Ammonia is an example of a molecule with trigonal pyramidal geometry. It consists of one nitrogen atom bonded to three hydrogen atoms. The nitrogen atom has a lone pair of electrons, resulting in a pyramid shape. This molecular structure gives ammonia its distinct properties and behavior.

Phosphine (PH3)

Phosphine is another molecule that exhibits trigonal pyramidal geometry. It contains one phosphorus atom bonded to three hydrogen atoms. Similar to ammonia, phosphine also has a lone pair of electrons on the phosphorus atom, leading to its pyramid-like shape.

Hydrogen Sulfide (H2S)

Hydrogen sulfide adopts a trigonal pyramidal structure due to its two bonding pairs and two lone pairs on the sulfur atom. The presence of these electron pairs creates repulsion, causing the molecule to form a pyramid shape.

Water (H2O)

Water can be considered as having a distorted tetrahedral or bent-trigonal-pyramid structure. Although it doesn’t strictly fit into the category of trigonal pyramidal molecules, it possesses some similarities. Water consists of two hydrogen atoms bonded to an oxygen atom, with two lone pairs on the oxygen atom. These lone pairs create repulsion and give water its bent shape.

Trigonal pyramidal molecules are found in many chemicals. They have special characteristics because of their shape. Knowing their structure helps us understand how they act in reactions.

Exploring the Structure of Ammonia:

Ammonia (NH3) is a molecule consisting of one nitrogen atom bonded to three hydrogen atoms. It has a tetrahedral electron domain arrangement, meaning that the four electron domains around the central nitrogen atom are arranged in a tetrahedral shape.

Nitrogen and Hydrogen Bonding

The structure of ammonia is unique because it adopts a trigonal pyramidal shape. This is due to the presence of one lone pair on the nitrogen atom. The three hydrogen atoms bond with the nitrogen atom, resulting in a triangular base and one lone pair extending above it.

Bond Angle and Polarity

The bond angle between the hydrogen atoms in ammonia is approximately 107 degrees. This angle arises from the repulsion between electron pairs, including both bonding and nonbonding pairs, within the molecule.

Ammonia is considered a polar molecule because of its uneven distribution of charge. The nitrogen atom carries a partial negative charge due to its higher electronegativity, while each hydrogen atom has a partial positive charge. This polarity gives ammonia unique chemical properties.

Significance in Chemistry

Knowing the structure of ammonia is important in chemistry. It helps with making things and is used in fertilizers because it has nutrients.


Learning about the differences between trigonal pyramidal and trigonal planar arrangements can help us understand how molecules are structured. We looked at examples of molecules with these arrangements, like ammonia. By comparing them, we learned more about their special features. Knowing about molecular geometry is important in fields like chemistry, biology, and materials science. Whether you’re a student or a professional, understanding trigonal pyramidal geometry can improve your knowledge and problem-solving skills.


What determines whether a molecule adopts a trigonal pyramidal shape?

The shape of a molecule is determined by the arrangement of its atoms and lone pairs. In the case of trigonal pyramidal geometry, it occurs when a central atom is bonded to three other atoms and has one lone pair. The presence of this lone pair causes the molecular structure to adopt a pyramid-like shape.

How does the presence of a lone pair affect the bond angles in trigonal pyramidal molecules?

The presence of a lone pair in trigonal pyramidal molecules affects the bond angles. In an idealized trigonal pyramidal structure, where all atoms are identical, the bond angles should be 107 degrees. However, due to electron repulsion caused by the lone pair, these angles can deviate slightly from the idealized value.

Can you provide examples of compounds that exhibit both trigonal planar and trigonal pyramidal geometries?

Certainly! One example is ammonia (NH3), which exhibits a trigonal pyramidal geometry due to its central nitrogen atom being bonded to three hydrogen atoms and having one lone pair. On the other hand, boron trifluoride (BF3) showcases a trigonal planar geometry as it lacks any lone pairs on its central boron atom.

Are there any exceptions or variations to the idealized bond angles in trigonal pyramidal molecules?

Yes, there can be exceptions or variations in bond angles for certain compounds. For instance, water (H2O) adopts a bent or V-shaped molecular structure rather than an idealized trigonal pyramidal shape due to two lone pairs on its oxygen atom. This results in slightly smaller bond angles compared to an ideal 107 degrees.

Is ammonia the only molecule with a trigonal pyramidal shape?

No, there are several other molecules that exhibit a trigonal pyramidal shape. Some examples include phosphine (PH3), hydrogen sulfide (H2S), and water (H2O).

What causes a molecule to adopt a trigonal pyramidal geometry?

A molecule adopts a trigonal pyramidal geometry when it has one central atom bonded to three surrounding atoms and one lone pair of electrons on the central atom. This arrangement minimizes electron repulsion and maximizes stability.

Can molecules with trigonal pyramidal geometry exhibit polarity?

Yes, molecules with trigonal pyramidal geometry can exhibit polarity. This depends on the electronegativity difference between the central atom and the surrounding atoms. For example, ammonia (NH3) is a polar molecule due to the electronegativity difference between nitrogen and hydrogen.

Are there any biological molecules that have a trigonal pyramidal shape?

Yes, several biological molecules adopt a trigonal pyramidal shape. One notable example is the amino acid serine, which plays a crucial role in protein synthesis and enzymatic reactions.

How does the bond angle in a trigonal pyramidal molecule compare to other geometries?

In a trigonal pyramidal molecule, the bond angle between the central atom and its surrounding atoms is approximately 107 degrees. This differs from other geometries such as linear (180 degrees) or tetrahedral (109.5 degrees).

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