Chemical Bonding And Molecular Structure

Understanding chemical bonding, molecular structure, and molecular formulation are considered the most basic and crucial knowledge when studying the subject of chemistry as a whole. In this, we shall discuss some essential sub-topics in chemical bonding to know how it works.

When we talk about studying chemical bonding and molecular structure in chemistry, we need to understand some methods/topics to get the gist of it. This article shall discuss the three main issues considered the backbone of bonding and structuring at an atomic level.

VSEPR Theory

Valence Shell Electron Pair Repulsion theory or better known as the VSEPR theory, is one of the molecular structure models that is used to predict the structural geometry of molecules in chemistry. This is done by putting up a central atom and then surrounding them with single electron pairs that help complete the octets for the central and attaching atoms, thus achieving a balanced state.

While strong bonds are created, this results in a phenomenon called electron-electron repulsion, which occurs due to the same charge repulsion in the valence electron of the molecule formed. This typically results in a unique geometric arrangement among the atoms, which decreases the overall energy of the molecule formed. The geometries of molecules vary depending on the number of lone pairs in that molecular structure, as they are ultimately responsible for the repulsion in the valence electrons.

Here are some geometries according to the number of electron groups and lone pairs of electrons in a molecule:

Hybridization theory

The hybridization theory or orbital theory is as the name suggests. In this chemical bonding theory, we make a completely new hybrid orbital between any 2 atoms in a molecule by joining orbitals of the same 2 atoms. The 2 joining orbitals are generally of comparable energy level, but the new hybridised orbital has its unique energy state and orbital shape. By going through this hybridisation process, the hybridised atoms expand their energy and create a more stable molecular form having its molecular formula.

Hybridization of the central atom in CH4  is sp3 as one s subshell and 3 p subshells participated in hybridization and generated four  sp3 hybrid orbitals having the same energy.

Just like we discussed in the VSEPR theory of chemical bonding, the hybridization theory also puts forward its types:

• sp hybridization

1. A single orbital of s and a single orbital of p in the same host shell of a given atom combine to form 2 new equivalent orbitals.
2. Half s and half p nature of the formed molecule can be observed.
3. It is also known as diagonal hybridization.

• sp2 hybridization

1. Forms when 2 orbitals of p and a single orbital of s of the same atom-shell combine.
2. p characteristic is 66.66%, while s characteristics account for 33.33% 
3. Also known as trigonal hybridization.

• sp3 hybridisation

1. Forms when 3 orbital of p and a single orbital of s of the same atom-shell combines.
2. p characteristic is 75%, while the s characteristic accounts for 25%.
3. Also known as tetrahedral hybridization.

• sp3d hybridization

1. Forms with the mixture of 1 s, 3 p, and 1 d orbital in the same energy.
2. s and d characteristic accounts for 20% each while p accounts for 60%.
3. Also known as trigonal bipyramidal hybridization. 

• sp3d2 hybridization

• Forms with the mixture of 1 s, 2 d, and 3 p orbitals of similar energy states.

• s characteristic accounts for 16.66%, d for 33.33%, while p accounts for 50% characteristic.
• Also known as octahedral hybridization.

Molecular Orbital Theory

The molecular orbital theory is another vital addition to the theories describing features and behaviours of how atoms interact when they form a chemical bond forming a new molecule.

The molecular bond theory came into existence when previously discussed theories like VSEPR and hybridization failed to incorporate or instead explain how two atoms interact when the electron is not assigned to either of the atoms.  It explains the movement of electrons as waves which introduces the concepts like constructive/deconstructive interferences by calculating net attractive/repulsive forces between the atoms, respectively.

We can understand the molecular orbital theory by discussing its salient features.

1. In MOT, two atoms come together and form a bond. This leads to the mixing of their nucleus, thus creating a single molecular orbital.
2. Only the same energy and symmetry orbitals can combine to form a new molecular orbital.
3. Once a new molecule is formed, it has its unique properties, which may be completely different from the individual parent atom (who loses their identity).
4. Unlike atoms, which have a single nucleus, molecules have more than one nucleus, thus being polycentric.
5. The resulting molecule can be bonding, antibonding and nonbonding orbitals.
6. Bonding molecular orbitals generally have lower energy than antibonding molecular orbitals.
7. The MOT incorporates the concept of quantum mechanics and is thus explained with the help of wave function, where the molecular orbital is described with the use of quantum numbers.
8. The electron of the atoms involved in forming a new molecular orbital follows Aufbau, Pauli, and Hund’s principles.

Conclusion

Chemical bonding and molecular structures are considered one of the most critical concepts in chemistry. The interaction between the atoms and the resulting molecules tells us a lot about the day-to-day things we interact with within our daily lives.