Types of molecular geometry

Atoms bond together to form molecules, and each element’s atoms are different. They are of different sizes, they have different electron configurations, and their chemical behaviour means that they act differently inside a molecule. In this way, the variety of atoms and the behaviour of their electrons means that molecules form in certain ways, and their shapes can be predicted using molecular geometry.

 By seeing atoms as balls and atomic bonds as sticks, “toothpick models” that approximate molecular structures can be created.

The Valence Shell Electron Pair Repulsion theory, or VSEPR, is used to describe molecular geometry. While electrons can dwell in pairs within an orbital, because they are all negatively charged, they will repel with one another. Counting the valence electrons in a molecule and accounting for how they prefer to be as far apart as possible helps explain most molecular geometry. 

As molecules get larger, they acquire stable configurations in which electron pairs find a “comfortable” distance apart from one another, resulting in the formation of recognisable geometric forms.

Electron geometry vs. molecular geometry:

Electron geometry (or electron-domain geometry): The arrangement of electron domains around the central atom (lone pairs and bonds).Molecular geometry is the arrangement of bonded atoms (only bonds).

Prediction of molecular geometry:

1 possible molecular shape ⇒ linear (AX2) 

3 electron domains: trigonal planar electron geometry (angle = 120°)

2 possible molecular shapes ⇒ trigonal planar (AX3) & bent (AX2E) 4 electron domains: tetrahedral electron geometry (angle = 109.5°)

3 possible molecular shapes ⇒ tetrahedral (AX4)

4 possible molecular shapes ⇒ trigonal bipyramidal 

Conditions for a molecule to be polar:

It must contain polar bonds (bond dipoles)

The molecular geometry shouldn’t balance out the effect of polar bonds (by vector addition).

It’s conceivable for a molecule to have polar bonds without becoming polar. Its overall dipole moment is equal to 0.

How to determine molecular geometry?

1.Draw the Lewis structure of the compound

2. Count how many electron domains there are on the central atom.

3.Determine the electron geometry according to the VSEPR theory

4.Determine the molecular geometry by considering only the positions of the atoms.

Linear Geometry:

The linear molecular geometry defines the geometry around a central atom bonded to two other atoms (or ligands) placed at a bond-angle of 180°. Linear organic molecules, such as acetylene (HC≡CH), are often described by invoking sp orbital hybridization for their carbon centres.

Linear geometry occurs at centre atoms with two bonded atoms and zero or three lone pairs (AX2 or AX2E3) in the AXE notation, according to the VSEPR model (Valence Shell Electron Pair Repulsion model). Examples of neutral compounds are beryllium fluoride (FBeF) with two single bonds, carbon dioxide (O=C=O) with two double bonds, and hydrogen cyanide (HCN) with one single and one triple bond.AX2 molecules with linear geometry. Acetylene (HCCH) is the most important linear molecule with more than three atoms, with each of its carbon atoms acting as a core atom with a single hydrogen bond and a triple connection to the other carbon atom.

The centre atom’s five valence electron pairs form a trigonal bipyramid, with the three lone pairs filling the less congested equatorial locations and the two bound atoms occupying the two axial positions at opposite ends of an axis, resulting in a linear molecule, as represented by the VSEPR model.

Planar trigonometry:

A molecular geometry model with one atom in the centre and three atoms at the corners of an equilateral triangle, known as peripheral atoms, all on one plane is known as trigonal planar. 

In an ideal trigonal planar species, all three ligands are identical, and all bond angles are 120°.

These species are classified as part of the D3h point group.Molecules where the three ligands are not identical, such as H2CO, deviate from this idealised geometry. Examples of molecules with trigonal planar geometry include boron trifluoride (BF3), formaldehyde (H2CO), phosgene (COCl2), and sulphur trioxide (SO3) .

Tetrahedral:

Tetrahedral shapes are formed by, according to electron domain geometry and VSEPR theory, four “electron domains” (bonds or lone electron pairs around the central atom) (bonds or lone electron pairs around the central atom). The tetrahedron is generated by electrons repelling one another, resulting in the tetrahedron—the shape in which all of the electrons are as far apart as possible.

Examples include methane (CH4) and ammonium (NH+4).

Conclusion:

 A molecule’s form has a role in determining its properties.

Carbon dioxide, for example, is a linear molecule. CO2 molecules are nonpolar, which implies they won’t dissolve well in water (a polar solvent).

Other molecules are shaped differently. The structure of water molecules is bent. One reason water molecules are polar and have features like cohesion, surface tension, and hydrogen bonding is because of this.