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.

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?

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

Steric Number 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

Steric Number 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

Steric Number 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

Steric Number 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

Q1

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.

Q2

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)]

Q3

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.

Q4

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.

Q5

What will be the shape of a molecule having 5 electron pairs?

It will have a trigonal bipyramidal shape.

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