Atoms are rarely found as free and independent units but usually are linked or bonded to other atoms in some manner as a result of interatomic forces. These binding forces between the atoms are called chemical bonds.
These bonds are made up of attractive and repulsive forces that tend to hold atoms (ions, molecules) or atomic units at a different spacing such that the opposing forces just balance; and the process of holding them is called bonding.
According to the strength, chemical bonds are grouped into primary and secondary bonds.
The primary bonds are interatomic bonds, whereas the secondary bonds are intermolecular bonds.
The primary bonds are stronger than the secondary bonds.
The attractive forces between the atoms are basically electrostatic in origin and the classification of the different types of bonding is strongly dependent on the electronic structure of the atoms concerned, and hence directly related to the Periodic Table.
Types of Bonding in Solids
There are basically four classes into which the bonds can conveniently be divided, although the boundaries between them are not always distinct:
- Ionic Bonding,
- covalent bonding,
- metallic, or unsaturated covalent bonding, and
- van der Waals bonding.
The first three are primary bonds, whereas the fourth one is a secondary bond.
Primary Bonds in Solids
- It is the simplest type of primary bond.
- Electrons break free of atoms with excess in their valence shell, producing positive ions, and unite with atoms having an incomplete outer shell to form negative ions.
- The positive and negative ions have a natural attraction to each other, producing a strong bonding force.
- The below Figure shows the process for a bond between sodium and chlorine.
- In Ionic-type bonding, however, the atoms do not unite in simple pairs. All positively charged atoms attract all negatively charged atoms.
- Thus, for example, sodium ions surround themselves with negative chlorine ions and chlorine ions surround themselves with positive sodium ions
- The attraction is equal in all directions and results in a three-dimensional structure, rather than the simple link of a single bond.
- For stability in structure, the total charge neutrality must be maintained, thereby requiring an equal number of positive and negative charges.
- Some other examples are bonds in MgCl2, K2O, CuO, MoF2, etc.
- General characteristics of materials joined by ionic bonds include moderate to high strength, high hardness, brittleness, high melting point, and electrically insulating properties.
- In this type of bond, the atoms being linked find it impossible to produce a complete shell by electron transfer but achieve the same goal by electron sharing so that each attains a stable electronic structure.
- Moreover, the shared negative electrons locate between the positive nuclei to form the bonding link.
- The covalent bond is found in a wide variety of materials since it can be formed between atoms of the same or different elements.
- Some of the materials having covalent bonds are Cl₂, N₂, HF, diamond, etc.
- Like the ionic bond, the covalent bond tends to produce materials with high strength and high melting points.
- Atom movement within the material (deformation) requires the breaking of distinct bonds, thereby making the material characteristically brittle.
- Electrical conductivity depends upon bond strength, ranging from conductive tin (weak covalent bond) through semiconductive silicon and germanium to insulating diamond.
- Engineering materials possessing ionic and covalent bonds tend to be ceramic or polymeric in nature.
- This type of bond is characteristic of the elements having a small number of valence electrons, which are loosely held so that they can be easily released to the common pool.
- The bonding results when each of the atoms of the metal contributes its valence electrons to the formation of an electron cloud that pervades the solid metal.
- In the metallic bonds, the highly mobile free electrons account for the observed well as the opaque optical properties (electrons can absorb light radiation energies).
- Electrons provide necessary bonding forces.
- Bond strength and, therefore, the material strength vary over a wide range.
- Positive ions can move without breaking distinct bonds.
- Materials bonded by metallic bonds, therefore, can be deformed by atom movement mechanisms and produce a deformed material every bit as strong as the original.
- This is the basis of metal plasticity, ductility, and many of the shaping processes used in metal fabrication.
Secondary Bonds in Solids
van der Waals bond
- Weak or secondary bonds known as van der Waals forces can link molecules that possess a non-symmetric distribution of charge.
- Some molecules, such as hydrogen fluoride and water, can be viewed as electrical dipoles in that certain portions of the molecules tend to be more positive or negative than others (an effect referred to as polarization).
- The negative part of one molecule tends to attract the positive part of another to form a weak bond.
Another weak bond can result from momentary polarization caused by the random movement of the electrons and the resulting momentary electrical unbalance. This random and momentary polarization leading to attractive forces is called the dispersion effect.
The third type of weak bond is the hydrogen bridge, where a small hydrogen nucleus is simultaneously attracted to the negative electrons of two different atoms, thereby forming a three-atom link.
Such bonds play a significant role in the biological system, but all secondary bonds are rarely of engineering significance.