VALENCE SHELL ELECTRON PAIR REPULSION (VSEPR) THEORY VSEPR THEORY-INORGANIC CHEMISTRY

The VSEPR Theory-Inorganic Chemistry (Valence Shell Electron Pair Repulsion) model explains molecular shapes and geometries based on Lewis dot structures. This theory relies on the concept that valence electron pairs repel each other. As a result, a specific geometrical arrangement forms to minimize this repulsion. The model also considers the positions of both atoms and electron pairs. The arrangement of bonded electrons and lone pairs determines the molecule’s shape. However, the final shape is based primarily on the bonded electron pairs, not the lone pairs. Check the detail in VSEPR theory – Wikipedia. The following steps outline how to apply VSEPR geometry to a molecule:

1. Draw the Lewis structure of the molecule to assess its geometry.
2. Position the lone pairs (lp) and bonding pairs (bp) of electrons around the central atom.
3. The strength of electron-electron repulsions follows this order: lone pair-lone pair (lp-lp) > lone pair-bonding pair (lp-bp) > bonding pair-bonding pair (bp-bp).
4. When the central atom is linked to surrounding atoms with multiple bonds, repulsions follow the order: triple bond-single bond > double bond-single bond > single bond-single bond.

Here are the possible common shapes for a molecule or ion, ABn, where a central atom A is surrounded by n atoms of B, based on the VSEPR theory:

STRUCTURE OF BERYLLIUM CHLORIDE (BeCl2) VSEPR THEORY-INORGANIC CHEMISTRY

Beryllium chloride is a white, hygroscopic solid with the chemical formula BeCl2. The beryllium atom in BeCl2 has two bonding pairs of electrons and no lone pairs. To reduce repulsion between the bonding pairs, the electrons position themselves as far apart as possible, giving the molecule a linear shape. According to VSEPR theory, the bond angle between the chlorine atoms and beryllium (Cl-Be-Cl) is 180°.

STRUCTURE OF BORON TRICHLORIDE (BcL3) VSEPR THEORY-INORGANIC CHEMISTRY

Boron trichloride is a colorless gas with the chemical formula BCl3. The boron atom in this molecule has three bonding pairs of electrons, resulting in a trigonal planar structure with Cl-B-Cl bonds.

STRUCTURE OF WATER (H2O), AMMONIA (NH3) AND METHANE (CH4) 

Each of the water, ammonia and methane molecules has four electron pairs around the respective central atom. However, the number of bonding and lone pairs of electrons is different. Methane with four bonding pair of electrons has a tetrahedral geometry (H-C-H bond angle = 109.5°). Ammonia has three bonding pairs and a lone pair of electrons; therefore, the geometry reduces to trigonal pyramidal. Since, the lp-bp repulsion is stronger than the bp-bp repulsion; therefore, H-N-H bond angles of ammonia are contracted to 107°. In case of water, there are two lone pairs and two bonding pairs of electrons which surround the oxygen atom. Therefore, the geometry of molecule reduces to bent shape with H-O-H bond angle of 105°.

STRUCTURE OF NITRATE ANION (NO3-) VSEPR THEORY-INORGANIC CHEMISTRY

The central nitrogen atom bonds to one oxygen atom with a double bond, while it forms single bonds with two other oxygen atoms. Additionally, there are no lone pairs of electrons around the nitrogen atom. Therefore, VSEPR theory predicts that the nitrate ion adopts a trigonal planar geometry.

STRUCTURE OF PHOSPHORUS PENTACHLORIDE (PCL5) VSEPR THEORY-INORGANIC CHEMISTRY

The phosphorus pentachloride (PCl5) molecule contains five bonding pairs of electrons around the phosphorus atom, with no lone pairs on the central atom. Consequently, according to VSEPR theory, this configuration results in a trigonal bipyramidal structure for PCl5.

STRUCTURE OF XENON TETRAFLUORIDE (XeF4) 

The Lewis structure of xenon tetrafluoride (XeF4) shows the central xenon atom with four bonding pairs and two lone pairs of electrons. This arrangement changes the expected octahedral geometry to a square planar shape for XeF4.

LIMITATIONS OF VSEPR THEORY

While VSEPR theory provides valuable general predictions about molecular geometries, helping to forecast the shapes of most molecules and ions, it does have limitations. In certain cases, the predicted structures do not align with those determined through physical characterization, as explained below:

1. IF7 and TeF7- are isoelectronic with seven bonding pairs around their central atoms, leading VSEPR to predict a pentagonal bipyramidal geometry. However, VSEPR doesn’t account for different bond lengths between axial and equatorial positions. Physical studies show that axial bonds are slightly shorter than equatorial ones, and crystallographic data reveal that TeF7- deviates significantly from the predicted geometry, with equatorial fluorine atoms not in a single plane.
2. p-block vs. d-block molecules: While VSEPR theory works well for predicting the geometries of simple p-block molecules, it is not suitable for predicting the structures of d-block compounds.
3. Inert pair effect: VSEPR theory does not consider the inert pair effect, making it inaccurate for explaining the structures of molecules involving heavy elements from the periodic table. Crystallographic studies have shown that species like [SeCl6]2-, [TeCl6]2-, and [BrF6]- exhibit regular octahedral geometries, which cannot be justified by VSEPR theory.

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