What is Electron Pair Geometry?
Electron Pair Geometry determines the spatial arrangement of a molecule’s bonds and lone pairs. VSEPR theory is used to compute the geometry of molecules in accordance with the arrangement of electron pairs around the central atom.
According to VSEPR theory,
Electron pairs around the central atom repel each other and will, therefore, arrange themselves to be as far apart as possible from each other.
Table of Content
- How to determine Electron Pair Geometry?
- Total number of electron pairs and Electron Pair Geometry
- Shape of Molecules
- Difference between Electron Pair Geometry and Molecular Geometry
- Frequently Asked Questions – FAQs
How to determine Electron Pair Geometry?
The Electron Pair Geometry of a molecule is determined by the total number of electron pairs around a central atom. Electron pairs are the bonded electrons, lone pairs and single unpaired electrons.
Total number of electron pairs = ½ X [(number of electron pairs on central atom) + (number of monovalent atoms on the central atom) + (anionic charge) – (cationic charge)]
Once the total number of electron pairs is estimated, we can quickly assess the electron pair geometry of the molecule.
- We can determine the lone pairs by subtracting the total number of electron pairs with the number of atoms on the central atom.
The total number of electron pairs and Electron Pair Geometry
The total number of electron pairs helps estimate the Electron Pair Geometry.
|
S. No. |
The total number of electron pairs |
Electron Pair Geometry of the molecule |
|---|---|---|
|
1 |
2 |
Linear |
|
2 |
3 |
Trigonal Planar |
|
3 |
4 |
Tetrahedral, Square Planar |
|
4 |
5 |
Trigonal Bipyramidal |
|
5 |
6 |
Octahedral |
|
6 |
7 |
Pentagonal Bipyramidal |
Shape of Molecules
Electron Pair geometry helps in assessing the shape of the molecule.
Linear Molecule
A Linear molecule has two electron pairs around the central atom. They are arranged so that the repulsion between the electron pairs is minimum.
- Electron pairs are arranged at a 180° angle in a linear molecule.
Example: Carbon dioxide.
Total number of electron pairs = ½ X [(number of electron pairs on central atom) + (number of monovalent atoms on the central atom) + (anionic charge) – (cationic charge)]
Total number of electron pairs = ½ X [ 4 + 0 – 0 + 0]
Total number of electron pairs = 2
Trigonal Planar Molecule
A Trigonal Planar molecule has three electron pairs around the central atom. They are arranged so that the repulsion between the electron pairs is minimum.
- Electron pairs are arranged at a 120° angle in a trigonal planar molecule.
Example: Boron trifluoride.
Total number of electron pairs = ½ X [(number of electron pairs on central atom) + (number of monovalent atoms on the central atom) + (anionic charge) – (cationic charge)]
Total number of electron pairs = ½ X [3 + 3 – 0 + 0]
Total number of electron pairs = 3
Tetrahedral Molecule
A Tetrahedral molecule has four electron pairs around the central atom. They are arranged so that the repulsion between the electron pairs is minimum.
- Electron pairs are arranged at a 109.5° angle in a tetrahedral molecule.
Examples: Water, Ammonia and Methane.
Water molecule
Total number of electron pairs = ½ X [(number of electron pairs on central atom) + (number of monovalent atoms on the central atom) + (anionic charge) – (cationic charge)]
Total number of electron pairs = ½ X [6 + 2 – 0 + 0]
Total number of electron pairs = 4
Lone Pairs = Total number of electron pairs – Bond Pairs
Lone Pairs = 4 – 2
Lone Pairs = 2
Ammonia
Total number of electron pairs = ½ X [(number of electron pairs on central atom) + (number of monovalent atoms on the central atom) + (anionic charge) – (cationic charge)]
Total number of electron pairs = ½ X [5 + 3 – 0 + 0]
Total number of electron pairs = 4
Lone Pairs = Total number of electron pairs – Bond Pairs
Lone Pairs = 4 – 3
Lone Pairs = 1
Methane
Total number of electron pairs = ½ X [(number of electron pairs on central atom) + (number of monovalent atoms on the central atom) + (anionic charge) – (cationic charge)]
Total number of electron pairs = ½ X [4 + 4 – 0 + 0]
Total number of electron pairs = 4
Trigonal Bipyramidal Molecule
A Trigonal Bipyramidal molecule has five electron pairs around the central atom. They are arranged so that the repulsion between the electron pairs is minimum.
- In a Trigonal Bipyramidal molecule, three electron pairs are at the equatorial of the molecule arranged at 120° angle while two electron pairs are at the axis perpendicular to the equatorial plane at 180° angle.
Example: Phosphorus pentachloride
Phosphorus pentachloride
Total number of electron pairs = ½ X [(number of electron pairs on central atom) + (number of monovalent atoms on the central atom) + (anionic charge) – (cationic charge)]
Total number of electron pairs = ½ X [5 + 5 – 0 + 0]
Total number of electron pairs = 5
Difference between Electron Pair Geometry and Molecular Geometry
|
S. No. |
Electron Pair Geometry |
Molecular Geometry |
|---|---|---|
|
1 |
Electron Pair Geometry predicts the shape of a molecule by considering both lone pairs and bond pairs. |
Molecular Geometry predicts the shape of a molecule by considering bond pairs only. |
|
2 |
The total number of electron pairs is calculated to determine the molecule’s shape. |
The number of bonding electrons pairs is calculated to determine the molecule’s shape. |
|
If there are no lone pairs, the electron pair geometry and molecular geometry will be identical. |
||
Frequently Asked Questions on Electron Pair Geometry
What is electron pair geometry?
Electron Pair Geometry determines the spatial arrangement of a molecule’s bonds and lone pairs. VSEPR theory is used to compute the geometry of molecules in accordance with the arrangement of electron pairs around the central atom.
How can we determine the electron pair geometry?
We determine the electron pair geometry by calculating the number of electron pairs around a central atom. Electron pairs are the bonded electrons, lone pairs and single unpaired electrons.
Total number of electron pairs = ½ X [(number of electron pairs on central atom) + (number of monovalent atoms on the central atom) + (anionic charge) – (cationic charge)]
Are electron pair geometry and molecular geometry the same?
No, electron pair geometry and molecular geometry are not the same. Electron Pair Geometry predicts the shape of a molecule by considering both lone pairs and bond pairs. In contrast, Molecular Geometry predicts the shape of a molecule by considering bond pairs only.
Can molecules have identical electron pair geometry and molecular geometry?
Yes, if the molecules have no lone pairs, they will have identical electron pair geometry and molecular geometry.
Example: Methane has a tetrahedral electron pair and molecular geometry.
What will be the shape of a molecule having 5 electron pairs?
It will have a trigonal bipyramidal shape.
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