Quantum mechanical approach to covalent bonding

Introduction

The matter is composed of molecules, and many atoms join together using interatomic linkages known as “bonding” or “chemical bonding”. The chemical bonding helps in holding together the atoms forcefully to make compounds, or we can say molecules. To understand the concept of Chemical bonding, one must be aware of some important terms. The terms are as follows:

• Bond – it describes the link or bridge or forces between two or more atoms that holds/joins them
• Intermolecular forces – it describes the forces of attraction or repulsion between two or more molecules
• Intramolecular forces – it describes the forces occurring inside a molecule
• Valence electrons – The free electrons present in the valence shell of an atom, involved in the formation of a chemical bond between two atoms is, called valence electrons
• Octet rule – the word “octet” means eight. An atom is highly stable when it has 8 electrons in its valence shell

Types of Bonding:

Chemical bonding is the binding between two or more molecules or atoms to make a compound. It is further classified into two types depending upon the strength of bonds between them these are: 

• Strong intermolecular forces
• Weak intermolecular forces

The strong intermolecular forces include the bonds which have strong forces of attraction or repulsion between them and can not be disrupted easily such bonds includes covalent bonds and ionic bonds

The weaker intermolecular forces include the forces which have weak bonding and can be easily disrupted. These include Hydrogen bonding, dipole-dipole interactions and London dispersion forces.

What is a covalent bond:

A Covalent bond is formed between two atoms having the same or equal electronegativity. It formed mostly between two nonmetals. It is formed by sharing a pair of electrons and such types of molecules have a net neutral charge on them.

Example – Ammonia, Nitrogen, Hydrogen, Water, Chlorine.

1.Ionic bond:

Atoms that have large differences in their electronegativity form the ionic bond. The gain or loss of ions forms ionic bonds. The molecules that have a positive charge in them are called cations, and the molecules carrying negative charge are known as anions. This type of bond is formed typically between a metal and non-metal.

Example – NaCl, NaF

2.Polar covalent bond:

These are types of bonds where electrons are spread unevenly. Such bonds are intermediate between ionic and covalent bonds, i.e. they are neither purely covalent nor purely ionic.

Example – Hydrogen Fluoride, Sulphur dioxide.

3. Covalent bond and The Quantum mechanics:

How electrons behave in a molecule or atom is explained by the ideas of quantum mechanics The term “Quantum mechanics” describes how the atomic or subatomic particles (electrons, protons, neutrons, quarks and gluons) behave, what are their nature and their physical properties. It was first Coined by a group of Scientists including Max Born, Werner Heisenberg and Wolfgang Pauli in the 1920s. It was observed that both matter and radiation have waves as well as particle nature. So, assuming that matter has wave-like properties and radiations have particle properties led to the development of the concept of Quantum Mechanics.

Formulas for quantum mechanics

• Photoelectric equation: Kmax = hf + Φ
• Dipole moment potential: U=-μB=-μzB
• Wavefunction probability density: ρ=|Ψ|²=Ψ∗Ψ

4. Quantum mechanical concept:

Kinetic energy is decreased by delocalisation of electrons: with an increase in the bulk distribution of an electron,and the Kinetic energy w.r.t electron decreases. It can be written in an equational form as below:

KE = p²/2m 

• The wave function for an electron is used to calculate the potential energy of interaction between the electron and other charges

• When the electron concentrates near the nuclei, the potential energy reduces, and when the electron is delocalised, the kinetic energy is reduced

So, E = KE + PE

Bond formation occurs when the electron distribution of molecular orbitals yields lower energy than the atomic orbitals—for example, covalent bonds.

5. Stability of bonds:

1. Covalent or Ionic interactions allow an atom to gain electrons in its valence shell. An atom is most stable when it has eight electrons in its valence shell, which satisfy the octet rule and hence makes it stable.
2. The atomic property of an atom helps in determining the property of the macromolecule (build of that particular atom). For example, Long-chain polymers involving covalent interactions are strong and difficult to disrupt, thus increasing the durability of the compound. Whereas on the other hand, Small covalent compounds are soft and malleable.

Conclusion:

Ions are formed due to large electronegativity differences in the transfer of electrons. Then the ions of opposite charges attract each other to form ionic bonds, usually formed between a metal and nonmetal. Covalent bonds are formed due to the equal sharing of electron pairs of atoms having nearly the same or equal electronegativity, usually formed between nonmetals. Elements obey the octet rule by gaining or losing valence electrons to become highly stable. Chemical bonds can be Single, Double or triple bonds depending upon the number of electron pairs involved. If one electron pair is involved, it forms a single bond. If two electron pairs are involved in bond formation, then the double bond is formed, and if triple electron pairs are involved in bond formation, then it is called a triple bond.