What is electron pair geometry?
Electron pair geometry determines the spatial arrangement of bonds and lone pairs in a molecule. VSEPR theory is used to calculate the geometry of molecules based on the arrangement of electron pairs around the central atom.
According to VSEPR theory,
Electron pairs around the central atom repel each other. and therefore they will be arranged to be as far apart as possible from each other.
Table of Contents
- How to determine the geometry of the electron pair?
- Total number of electron pairs and electron pair geometry
- Shape of molecules
- Difference between electron pair geometry and molecular geometry
- Frequently Asked Questions – Frequently Asked Questions
How to determine the electron pair geometry?
The electron pair geometry of a molecule is determined by the total number of pairs of electrons around a central atom. The electron pairs are the bonded electrons, the lone pairs, and the single unpaired electrons.
Total number of electron pairs = ½ X [(number of electron pairs on the central atom) (number of monovalent atoms in the central atom) ( anionic charge) – (cationic charge)]
Once the total number of electron pairs has been 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 from the number of atoms in the central atom.
The total number of electron pairs and the geometry of electron pairs
The total number of electron pairs helps to estimate the geometry of the electron pair.
S. No.
The total number of electron pairs
Electron pair geometry of the molecule
1
2
Linear
2
3
Trigonal Plane
3
4
Tetrahedral Square Plane
4
5
Trigonal Bipyramidal
5
6
Octahedral
6
7
Bipyramidal pentagonal
Shape of molecules
Geometry The pair of electrons helps in evaluating the shape of the molecule.
Linear Molecule
A linear molecule has two pairs of electrons around the central atom. They are arranged so that the repulsion between the electron pairs is minimal.
- Electron pairs are arranged at an angle of 180° in a linear molecule.
Example: Carbon dioxide.
Total number of electron pairs = ½ X [(number of electron pairs in the central atom) (number of monovalent atoms in 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 pairs of electrons around the central atom. They are arranged so that the repulsion between the electron pairs is minimal.
- The electron pairs are arranged at an angle of 120° in a trigonal planar molecule.
Example: boron trifluoride.
Total number of electron pairs = ½ X [(number of electron pairs on the central atom) (number of monovalent atoms on the central atom) (charge anionic) – (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 pairs of electrons around the central atom. They are arranged so that the repulsion between the pairs of electrons is minimal.
- The pairs of electrons are arranged at an angle of 109.5° in a tetrahedral molecule.
Examples: Water, Ammonia and Methane.
Water molecule
Total number of electron pairs = ½ X [(number of electron pairs on the central atom ) (number of monovalent atoms in 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 – Bonding pairs
Lone pairs = 4 – 2
Pairs solitary = 2
Ammonia
Total number of electron pairs = ½ X [(number of electron pairs on central atom) (number of monovalent atoms on central atom) ( anionic charge) – (cationic charge)]
Total number of electron pairs = ½ X [5 3 – 0 0]
Total number of p pairs of electrons = 4
Lone Pairs = Total Number of Electron Pairs – Bonding Pairs
Lone Pairs = 4 – 3
Lone Pairs = 1
Methane
Total number of electron pairs = ½ X [(number of electron pairs on the 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 pairs of electrons around the central atom. They are arranged so that the repulsion between pairing of electrons is minimal.
- In a trigonal dipyramidal molecule, three pairs of electrons are on the equatorial of the molecule arranged at an angle of 120°, while two pairs of electrons are on the equatorial side of the molecule. the axis perpendicular to the equatorial plane at an angle of 180°.
Example: Phosphorus pentachloride
Phosphorus pentachloride
Total number of pairs of electrons = ½ X [(number of electron pairs in central atom) (number of monovalent atoms in 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 Metric Molecular Geometry
S. No.
Electron pair geometry
Molecular geometry
1
Electron pair geometry predicts the shape of a molecule considering both lone pairs and bond pairs.
Molecular geometry predicts the shape of a molecule considering only bond pairs.
2
The total number number of pairs of electrons is calculated to determine the shape of the molecule.
The number of pairs of bonding electrons is calculated to determine the shape of the molecule.
If there are no pairs solitaires, the electron pair geometry and the molecular geometry will be identical.
.