Geometry of Molecules

The three-dimensional architecture or distribution of atoms in a molecule is characterized as molecular geometry or molecular structure. Considering a compound’s molecular structure can help to determine the compound’s polarities, reactivity, phase of matter, colour, magnetism, and bioactivity. We must first learn about the Lewis electron-dot structure in order to determine the shapes of molecules. Despite the fact that the Lewis theory does not dictate molecular shapes, it is the first step in predicting them. The Lewis framework assists in the determination of bond pairs and lone pairs. The molecular geometry and electron-group geometry then are computed using the valence-shell electron-pair repulsion (VSEPR) theory and the Lewis configuration.

A brief outline

Various spectroscopic and diffraction approaches can be used to determine molecular geometry. The specifics of the vibrational and rotational absorbance detected by IR, microwave, and Raman spectroscopy can provide information about the molecular shape. The distance between nuclei and the concentration of electron density can be used to determine the molecular structure of crystalline solids using X-ray crystallography, neutron diffraction, and electron diffraction. Small molecules in the gas phase can be studied using gas electron diffraction. Complementary information such as relative distances, dihedral angles, angles, and connections can be determined using NMR and FRET approaches.

Because the molecule structure is averaged over much more accessible geometries at higher temperatures, molecular geometries are best determined at low temperatures. Larger molecules frequently exist in numerous stable configurations (conformational isomerism) on the potential energy surface that is near in energy. Geometries can also be determined with high precision using ab initio quantum chemistry methods. As a solid, in solution, or as a gas, the molecular formula can alter.

Important concepts

Molecular shape

Predicting Molecule Shapes: In order to determine the form of a molecule, the steps below must be followed.

• As the central atom, the lowest electronegative atom should be chosen
• It is important to calculate the total number of electrons in the central atom’s outer shells.
• It is necessary to count the total number of electrons from other atoms that are used in bonding with the central atom.
• To get all the valence shell electron pair numbers, or VSEP numbers, add these two numbers simultaneously.

A molecule with a Linear Shape:

This form of the molecule has two spots in the valence shell of the central atom. They should be organized in such a way that repulsion is reduced to the bare minimum. BeF₂ is an example.

Trigonal Planar Shape Molecule:

Three molecules are bonded to a central atom in this form of the molecule. The electrons are organized in such a way that repulsion between them is reduced. BF₃ is an example.

Tetrahedral Molecule Shape:

Atoms in two-dimensional molecules reside in the same plane, and when these criteria are applied to methane, we get a square planar geometry with a bond angle of 900. When all of these parameters for a three-dimensional structure are taken into account, we get a tetrahedral molecule with a bond angle of 109ͦ°28′ between H-C-H.

Molecule Shape: Trigonal Bipyramid

Let’s look at the case of PF_5. The uniform distribution of electrons towards the corner of a trigonal pyramid can reduce repulsion here. Three places along the molecule’s equator make up the trigonal bipyramid. The two places are parallel to the equatorial plane on an axis.

H₂O geometry

1. Water has a basic Lewis structure, making it one of the easiest chemical substances to comprehend. While we’ve always known that chemistry is all around us, we had no idea that water indeed has chemical formulas when we were kids. Water is made up of 2 and 1 oxygen atom and has the chemical formula H₂O.
2. Dihydrogen monoxide is another chemical name for this molecule. Any molecule’s molecular geometry is determined by its Lewis structure, atom arrangement, and electrons. The oxygen atom makes 2 individual sigma bonds with the hydrogen atoms in the H₂O molecule.
3. Despite the fact that these Hydrogen atoms are symmetrically positioned in the plane, the two lone pairs of electrons on the Oxygen atom force these atoms apart. The arrangement of atoms is deformed because the repulsive forces of lone pairs are greater than the repulsive forces of bonded pairs.
4. As a result, the water molecule’s molecular geometry is angular or V-shaped, and this bond geometry is also known as deformed tetrahedron geometry.

SF₆ geometry

Sulphur is in the centre of the Sulphur Hexafluoride molecule, with the fluorine atoms placed symmetrically around it. The atoms are arranged in an octahedral arrangement, giving SF₆ is octahedral molecular geometry.

SF₆ shape

We may say that the SF₆ molecule seems to have an octahedral form because it has eight sides when looking at its chemical geometry. The centre atom, on the other hand, binds with 6 Fluorine atoms, giving SF₆ is octahedral structure.

The molecule SF₆ is a nonpolar one. The bond dipoles are eliminated when six fluorine atoms are placed symmetrically around the sulphur atom, according to the VSEPR theory. It’s a nonpolar molecule as a result of this. It also has the same features as nonpolar molecules and is soluble in nonpolar organic solvents but not in water.

Significance of resonance in NEET exam

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Also read: Valence Bond Theory

FAQs (Frequently asked questions)

What are some of the benefits of the VSEPR theory?

This theory can be used to precisely anticipate the shapes of molecules in a variety of compounds. It's easier to grasp a molecule's reactions once you understand its shape.

Is SF6 Gas Environmentally Harmful?

This gas has no effect on the ozone layer and does not pollute the air. SF6 is, however, over 24,000 times more efficient at trapping heat than carbon dioxide (CO2), making it a powerful greenhouse gas. That's why it's critical to keep an eye on the SF6 level in electrical component gears for any leakage.