What is Chemical bonding?
Chemistry deals with the transformation of substances and the reason behind those transformations. So why does any substance or compound undergoes transformation?
The answer to this question is hidden in the concepts of chemical bonding. Anything in the universe tries to become stable and they become stable by losing energy. So the main aim or the reason behind most of the chemical processes is to lose energy and to become stable. Whenever matter interacts with another form of matter, a force is exerted on one by the other. Now depending upon the nature of force, their energy will change. If the force is attractive in nature then the energy decreases and if it’s repulsive in nature then the energy increases. If the force is attractive, it binds the two atoms together and the force is known as a chemical bond. Thus the chemical bonding can be defined as follows:
The attractive force which holds various constituents (atom, ions, etc.) together and stabilizes them by the overall loss of energy is known as chemical bonding.
Since the formation of chemical compounds takes place as a result of the combination of various atoms in different ways, it raises many questions. Why do some atoms combine while certain others do not? How to determine which pair of atoms will combine and which will not? The underlying principle to unhide the answers to the above questions lies in the fact that there should be a proper attractive force between the elements to combine.
The answers to few of the questions are as follows:
Why do atoms combine?
Atoms combine together to lose their energy so as to become stable.
Why do some atoms combine while certain others do not?
Some atoms combine together while others do not because whenever there is an attractive force which leads to lowering of energy, a compound is formed. Whereas whenever there is a repulsive force, it leads to increase in overall energy of the system. Thus the compound formation doesn’t take place.
How to determine which pair of atoms will combine and which will not?
To determine which pair of atoms will combine and which will not, we will combine our understanding of the atomic structure, the electronic configuration of elements and the periodic table. We will then use that theory to determine the possibilities of combination of two atoms.
With the epoch of time, many scientists tried to explain the formation of chemical bonds in terms of electrons or other reasons. Most of these attempts failed to explain one function or other of the compounds formed due to the bonding between elements. In their attempt to become stable, atoms share, gain or lose electrons. There are basically 4 types of bonding methods by which atoms gain stability. They are:
Kössel and Lewis were first to explain the formation of chemical bonds successfully in the year 1916. They explained the chemical bonding on the basis of inertness of noble gases.
- An atom can be viewed as a positively charged ‘Kernel’ (the nucleus plus the inner electrons) and the outer shell.
- The outer shell can accommodate a maximum of eight electrons only.
- The eight electrons present in the outer shell occupy the corners of a cube which surround the ‘Kernel’.
- The atoms having octet configuration that is 8 electrons in the outermost shell thus symbolize a stable configuration.
- Atoms can achieve this stable configuration by forming chemical bonds with other atoms. This chemical bond can be formed either by gaining or losing an electron(s) (NaCl, MgCl2) or in some cases due to sharing of an electron (F2).
- Only the electrons present in the outer shell (also known as valence electrons) take part in the formation of chemical bonds. He used specific notations better known as Lewis symbols to represent valence electrons. Lewis symbols for lithium (1 electron), oxygen (6 electrons), neon (8 electrons) are given below:
Here, the number of dots that surround the respective symbol represents the number of valence electrons in that atom.
- Generally, the valency of an element is either equal to the number of dots in Lewis symbol or 8 minus the number of dots or valence electrons.
- Noble gases separate the highly electronegative halogens and the highly electropositive alkali metals.
- Halogens can form negatively charged ions by gaining an electron. Whereas alkali metals can form positively charged ions by losing an electron.
- These negatively charged ions and positively charged ions have a noble gas configuration that is 8 electrons in the outermost shell. The general electronic configuration of noble gases (except helium) is given by ns2np6.
- As unlike charges attract each other these unlike charged particle are held together by a strong force of electrostatic attraction existing between them. For example, MgCl2, the magnesium ion, and chlorine ions are held together by force of electrostatic attraction. This kind of chemical bonding existing between two unlike charged particles is known as electrovalent bond.
Thus by the points stated out by Kössel and Lewis, the ionic bond theory was developed. The main postulates of this theory are:
- Ionic bonding involves the transfer of electrons between atoms wherein one atom loses an electron and the other atom gains an electron.
- When an electron transfer occurs, one atom has a negative charge making it the anion and the other atom has a positive charge making it the cation.
- The ionic bond gains in strength as how opposite charges attract.
- Covalent bonding is the most common method of bonding seen in compounds containing carbon, basically organic compounds.
- A Covalent bond indicates the sharing of electrons between atoms.
- A new orbit is formed by the shared pair of electrons which extends around the nuclei of both the atoms creating a new molecule.
Polar bonds and hydrogen bonds are actually secondary types of covalent bonding.
- A covalent bond can be of two types:
- Polar Bond
- Non-polar Bond
- In a Polar bond, electrons are shared unequally and they tend to be closer to one atom than the other.
- Due to this uneven distance between the electron and atom, there is charge difference created in the different parts of the atom.
- One end of the molecule will be slightly positively charged and one end slightly negatively charged.
- Water is an example of a polar molecule.
- A hydrogen bond is a weaker form of bonding when compared to ionic and covalent bonding.
- Hydrogen bonding is a type of polar covalent bonding (O-H bonding) wherein the hydrogen has a slightly positive charge. This means that electrons are pulled more towards the other element.
- This will create a tendency for the hydrogen to be attracted towards the negative charges of any neighboring atom.
- Hydrogen bonds are responsible for many of the favorable characteristics of the substances around us like the structure of DNA, proteins and the properties of water.
There is one more kind of weak form of bonding known as London dispersion forces.
London dispersion forces
- London dispersion forces occur due to the temporary imbalances of charge which occur on the inside of an atom.
- Since electrons are in constant motion, there is always a change in the concentration of a charge of the atoms. This creates a temporary shift in the overall charge distribution of the atom. when this particle comes into contact with another, the temporary imbalance of charge will result in an attraction of positive and negative charges.
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