VSEPR Model

The VSEPR model calculates the valence shell electron bond pairs amongst atoms in a molecule or ion to predict 3-D molecular shape. As per the model, the electron pairs will arrange themselves in such a way as to reduce mutual repulsion. To put it another way, the electron pairs are as far apart as they can be.

VSEPR Shapes

For predicting and visualising molecular structures, the VSEPR model is beneficial. Linear, angled, tetrahedral, trigonometric planar, trigonometric pyramidal, trigonometric bipyramidal, t-shaped, octahedral, square pyramidal, square planar, disphenoidal (seesaw), and pentagonal bipyramidal are the structures.

The VSEPR structures are named after 3-D geometric shapes, such as the trigonal bipyramidal in this example. According to the VSEPR model, a trigonal bipyramidal molecule and five valence shell electron pairs, such as phosphorus pentachloride or PCl5 , looks like two (bi) linked triangular-base pyramids, with each atom being the vertex or corner of a triangular face. For more information, refer to the VSEPR model study material.

The shape of Five Molecules by VSEPR Theory

Linear Molecules

The two bonding orbitals of a triatomic molecule of type AX2 are 180 degrees apart, resulting in a molecule with linear geometry; examples are BeCl2 and CO2. The C-O bonds in the electron dot formula for carbon dioxide are double bonds. The core carbon atom is still connected to two other atoms, and the electron clouds connecting the two oxygen atoms are 180 degrees apart, according to the VSEPR model.

Trigonal Molecules

Three zones of electron density spread out from the core atom in an AX3 molecule, such as BF3. When the angle between any two of these is 120 degrees, the repulsion between them is minimum. All four atoms must be in the same plane to work; the resulting shape is trigonal planar, or simply trigonal.

Tetrahedrally-Coordinated Carbon Chains

A centre atom is connected to four additional atoms in an AX4 molecule, such as methane (CH4). Tetrahedral coordination is defined as the four analogous bonds pointing in four geometrically equivalent directions in three dimensions, corresponding to the four corners of a tetrahedron. Any two bonds will have a 109.5° angle between them. When the valence shell of the central atom contains nonbonding electrons, the bonding geometry is not tetrahedral.

Central Atoms with Five Bonds

Some of the elements in Group 15 of the periodic table combine to generate compounds of the type AX5. PCl5 and AsF5 are two examples of these chemicals.

The shape of molecules with a coordination number of 5 is a trigonal bipyramid, which is made up of two triangular-based pyramids linked base-to-base. Three chlorine atoms lie in the plane of the central phosphorus atom (equatorial locations) in a PCl5 molecule, whereas the other two atoms are above and below this plane (axial positions).

Octahedral Coordination

Six electron pairs in an AX6 molecule will strive to point toward the octahedron’s corners (two square-based pyramids joined base-to-base). A four-fold symmetry axis defines three equivalent planes, one of which is shaded in the diagram. There are no discrete axial and equatorial positions because all of the ligands are geometrically equal with bond angles of 90°.

Inorganic chemistry, the coordination number 6 is one of the most prevalent, especially in transition metal hydrates like [Fe(H2O)6]3+. 6-coordinate central atoms with one, two, or three lone pairs are well-known examples. Make sure you go through the study material notes on the VSEPR model to have an in-depth understanding.

Molecules containing lone electron pairs on the core atom VSEPR theory

Bent

A bent molecule is H2O, for example. When a molecule’s core atom has lone pairs, the lone pairs repel the bonds that are established in the core atom. The hydrogen bonds in a water molecule are forced downwards in 2-dimensional space by the lone pairs on the oxygen atom. The hydrogen atoms have a binding angle of 104 degrees.

Trigonal Pyramidal

A trigonal pyramidal molecule like NH3 is an example. Due to electron-electron repulsion, the lone pair on the central nitrogen atom forces the three N-H bonds downward in the ammonia molecule.

Seesaw

SF4 is an illustration of a seesaw-shaped molecule. Two of the S-F bonds in an SF4 molecule are across from one another in the equatorial plane. In three-dimensional space, the other two S-F bonds face away from each other. Because there is a lone electron pair on the centre sulphur atom, this allows the fluorine atoms to be the furthest apart from one another.

T-Shaped

T-Shaped molecules, such as BrF3, are an example. There are two lone pairs on the central atom of Bromine in a BrF3 molecule, forcing more intense electron-electron repulsion with the Br-F bonds than in other geometries. This form has 86.2-degree bond angles, which is only found in T-Shaped molecules.

Square Pyramidal

The geometry of a BrF5 molecule closely resembles that of an octahedral molecule. The sole difference is that one of the axial atoms is substituted with a lone electron pair in square pyramidal form.

AXE Method

The AXE approach is a method for representing molecular geometries differently. The A in the AXE model stands for the centre atom. The centre atom is represented by X, the number of single bonds connecting to it is represented by X, and the number of lone electron pairs on the central atom is represented by E.

Conclusion

The theory of valence shell electron pair repulsion predicts the geometry of molecules. It’s founded on the idea that electrons repel one another because of their comparable charges, and molecules build themselves in such a way that lone electron pairs are separated as much as possible. Most elementary molecules fall into one of 11 form groups, which we can accurately predict by knowing the number of valence electrons, identifying the core atom, and applying the VSEPR model.