Have you ever wondered why water has a bent shape or why carbon dioxide is straight like a line? The shape of a molecule isn’t random; it follows certain rules. One of the most useful theories that help us understand how molecules take shape is called the VSEPR Theory. 

VSEPR stands for Valence Shell Electron Pair Repulsion. It might sound complex at first, but don't worry it simply means that electron pairs around an atom want to stay as far away from each other as possible. Why? Because electrons are negatively charged and naturally repel one another.

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By using this theory, we can predict the 3D shape of molecules, which helps in understanding how they behave, react, and interact in real life from the air we breathe to the food we eat. Let’s explore what VSEPR theory is all about, how it works, and why it matters.

What is VSEPR?

Atoms bond chemically in order to form a molecule. At this point, two types of forces come into play. The attraction force between the nucleus and electrons and the repulsion force between the electrons. The shape of the atom is dependent upon the repulsion between the pairs of valence electrons. The Valence Shell Electron Pair Repulsion Theory (VSEPR) states that “Whenever there is a repulsion between the pairs of valence electrons in all atoms, the atoms will arrange themselves in a geometric shape so as to minimize the electron pair repulsion.” The aim is that the molecule must have minimum energy and maximum stability. In short, the repulsion between the pairs of valence electrons of all the atoms in the molecule determines the geometry of the resulting molecule. The VSEPR model can be used to predict and explain the shape of any molecule or polyatomic ion. This theory does not take into consideration the subtleties of orbital interactions that influence molecular shapes. As a result, the shapes of actual molecules are similar and not exact to the ones predicted by this theory. However, it is an advantage over the Lewis electron-pair approach as VSEPR more or less accurately predicts the three-dimensional structures of a large number of compounds with both metallic and non-metallic centres.

The postulates of the VSEPR theory are given below

• The total number of valence shell electron pairs of each atom in a molecule decides the shape of the molecule.
• The atoms will arrange themselves in a geometric shape so as to minimize the electron pair repulsion.
• For a polyatomic molecule with three or more atoms. One of the constituent atoms will take the central position and will be called the central atom. All other atoms belonging to the molecule will be linked to the central atom.
• The electron pairs are localized on the surface. The spherical surface enclosing them in such a way that the distance between them is maximized.
• If the bond pairs of electrons surround the central atom of the molecule then we get an asymmetrically shaped molecule.
• In VSEPR Theory, single bond, double bond and triple bond are all treated as one bond pair each.
• If both lone pairs and bond pairs of electrons surround the central atom of the molecule then we get a distorted molecule.
• The VSEPR theory can also be applied to get the different resonant structures for the same molecule.
• According to this theory, the molecule is shaped such that the strength of the repulsion is strongest in two lone pairs and weakest in two bond pairs.
• The increased energy results when electron pairs around the central atom are closer to each other because they will repel each other. The vice versa is also true. The low energy molecules have electron pairs far from each other. Hence, the repulsions between them will be less.

Know all about Solutions, its Components, Types, Properties here.

VSEPR Formula

Theoretical Shape of VSEPR 

Now, let us discuss each shape in detail:

Linear Shape of Molecule:

• The shape of a molecule with only two atoms is always linear.
• Linear shaped molecules are made from three atoms. One central molecule and two other atoms.
• In this type of linearly shaped molecule, the central atom has two places in the valence shell.
• In order to minimize the repulsion, the two atoms are pointing in the opposite direction making a 180-degree angle with each other.
• Example: BeF2

Trigonal Planar Shape of Molecule:

• Trigonal Planar shaped molecules are made from four atoms. One central molecule and three other atoms.
• In order to minimize the repulsion, the three molecules attached to a central atom like a tripod stand making 360-degree angles with each other on the corners of an equilateral triangle.
• Example: BF3

Also, check more about Quantum Numbers and Electronic Configuration, here.

Tetrahedral Shape of Molecule:

• Tetrahedral Planar shaped molecules are made from five atoms. One central molecule and four other atoms.
• In order to minimize the repulsion, three molecules are attached to a central atom like a tripod stand making 360-degree angles with each other and one angle is at the top, on the corners of a tetrahedron.
• Example: CH4

Trigonal Bipyramid Shape of Molecule:

• Trigonal Bipyramid shaped molecules are made from six atoms. One central molecule and four other atoms.
• In order to minimize the repulsion, three molecules are attached to a central atom and lie along the equator of the molecule while the two remaining atoms lie along an axis perpendicular to the equatorial plane, on the corners of a Trigonal Bipyramid.
• Example: PF5.

Octahedral Shape of Molecule:

• Octahedral shaped molecules are made from seven atoms, one central atom and six surrounding atoms
• In this shape, the central atom has six bonding pairs of electrons in its valence shell. To minimize repulsion between these electron pairs, the six atoms are arranged symmetrically around the central atom, pointing towards the corners of an octahedron
• It gives the molecule a perfect 90 degree bond angle between any two adjacent atoms

Examples- SF6 (Sulfur hexafluoride)

In SF6, sulphur is the central atom and is bonded to six fluorine atoms, forming a symmetric, octahedral shape.

Molecular Geometry of VSPER

• The actual shape depends on the number of bond pairs and lone pairs around the central atom.
• When we name the shape of a molecule according to the VSEPR theory we do not take the lone pair into consideration but the lone pair still exerts a repulsion and affects the shape of the molecule.
• Also, the repulsion involving a lone pair is greater than the repulsion involving a bond pair, hence the presence of a lone pair will decrease the bond angles as it will squeeze the bond pairs closer together.
• If the central atom is linked to similar atoms and it is surrounded by bond pairs of electrons only, the value of repulsions between them is exactly the same as a result the shape of the molecule is symmetrical. Such a molecule is said to have a regular geometry.
• If the central atom is linked to different atoms or is surrounded by a bond pair as well as a lone pair of electrons, the value of repulsions between them is not the same. As a result, the shape of the molecule is irregular. Such a molecule is said to have a distorted geometry.
• The exact shape of the molecule depends upon the total number of electron pairs present around the central atom.
• An estimate that is useful to determine the bond angle without memorising is for every lone pair, the bond angle will decrease by 2 degrees.
• For example, in the structure of the NH3 molecule, the presence of lone pairs causes slight distortion from 109°28’ to 107°48’.

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Applications Of VSEPR

VSEPR theory is considered as the most important chemical tool to predict molecular shape. The electron cloud residing on the central atom of a molecule rearranges them according to the way where they can face minimum repulsion. Due to this fact each molecule gets a specific three dimensional shape. VSEPR theory shows the route by combining some specific shapes which are directly related to the number of valence electrons (may be lone pair or bond pair). Thus, This theory has great importance in inorganic, organic as well as in stereochemistry.

Limitations of Lewis Structures

• According to the Lewis Structure, the same atomic composition can correspond to many acceptable Lewis structures.
• In the case of Resonance structure, it isn’t possible to produce two distinct Lewis electronic structures for the same atomic composition.
• It could not give any information about the release of energy during the structural formation of a covalent bond.
• It could not give the shapes of molecules.
• It could not give any information about the nature of attractive forces between the constituent atoms of a molecule.
• It could not give any information about the angles or lengths of bonds in 3D.

Check more important topics of Chemistry here.

VSPER Quick Summary Table

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