Difference Between Shape and Geometry of a Molecule

The key difference between shape and geometry of a molecule is that shape of a molecule is the structure of the molecule, excluding the lone pair on the central atom, whereas the geometry of a molecule describes the arrangement of lone pair and bond pair electrons around the central atom of the molecule.

We usually use the terms – shape and geometry of a molecule – interchangeably. However, these are two different terms for some molecules we know.

CONTENTS

1. Overview and Key Difference
2. What is Shape of a Molecule 
3. What is Geometry of a Molecule
4. Side by Side Comparison – Shape vs Geometry of a Molecule in Tabular Form
5. Summary

What is Shape of a Molecule?

The shape of a molecule is the structure of the molecule predicted using the bond electron pair on the central atom. In other words, the shape of a molecule is determined excluding the lone electron pairs of the central atom. The shape of the molecule can be predicted using the VSEPR model (valence shell electron pair repulsion model).

VSEPR model is the theory that determines the shape and geometry of a molecule. We can use this VSEPR model to propose a spatial arrangement for molecules having covalent bonds or coordination bonds. The basis of this theory is the repulsions between electron pairs in the valence shell of atoms. Here, we can find electron pairs in two types as bond pairs and lone pairs. There are three types of repulsion present in between these electron pairs; bond pair – lone pair repulsion, bond pair-bond pair repulsion, and lone pair- lone pair repulsion. For example, the shape of beryllium chloride molecule is predicted as follows:

The central atom is Be.
It has 2 valence electrons.
Cl atom can share one electron per atom.
Therefore, the total number of electrons around the central atom  = 2 (from Be) + 1×2 (from cl atoms) = 4
Therefore, the number of electron pairs around the Be atom           =  4 / 2    =    2
Number of single bonds present                                                              =  2
Number of lone pairs present                                                                   = 2 – 2     =    0
Therefore, the geometry of BeCl2 molecule is linear.

Figure 01: BeH2 molecule, which is similar to the shape of beryllium chloride molecule

What is Geometry of a Molecule?

The geometry of a molecule is the structure of the molecule, including both lone electron pairs and bond electron pairs of the central atom. Therefore, this term is different from the shape of a molecule because the shape of a molecule is determined using only the bond electron pair, excluding the lone electron pairs.

Figure 02: Geometry of a Water Molecule

There are different methods of determining the geometry of a molecule, such as various spectroscopic methods, diffraction methods, etc.

What is the Difference Between Shape and Geometry of a Molecule?

The key difference between shape and geometry of a molecule is that shape of a molecule is the structure of the molecule excluding the lone pair on the central atom whereas the geometry of a molecule describes the arrangement of lone pair and bond pair electrons around the central atom of the molecule. Usually, the terms shape and geometry of a molecule are used interchangeably because both these structures are typically the same for most molecules if there are no lone electron pairs on the central atom of the molecule.

Below infographic summarizes the differences between shape and geometry of a molecule.

Summary – Shape vs Geometry of a Molecule

The shape of a molecule is the structure of the molecule predicted using the bond electron pair on the central atom while the geometry of a molecule is the structure of the molecule including both lone electron pairs and bond electron pairs of the central atom. Thus, this is the key difference between shape and geometry of a molecule. Usually, the terms, shape and geometry of a molecule, are used interchangeably because both these structures are typically the same for most molecules if there are no lone electron pairs on the central atom of the molecule.

Reference:

1. “Geometry of Molecules.” Libretexts, Available here.

Image Courtesy:

1. “Linear molecule ex” By Meeshko – Own work (CC BY-SA 4.0) via Commons Wikimedia

2. “Water molecule dimensions” By Booyabazooka – Image:Water_molecule_dimensions.png (CC BY-SA 3.0) via Commons Wikimedia

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