How do Molecules Bond|Chemical Bond Form

How do Molecules Bond

Hello friends, welcome to our blog, today we are going to discuss formation of molecule on the basis of the valence bond theory(VBT).This theory  based on the concept of atomic orbital and also the orbital configuration of atoms.Whenever word chemical bond to hear bond it means tie something together and chemical means that is irreversible by physical means. Lets explain the formation of hydrogen(molecule) as an example. When the hydrogen atom combine to form molecule of hydrogen, energy is released. This shows that the energy of hydrogen molecule is less than as compared to the hydrogen atom.
In general, whenever atom combined to form molecule there is always decrease in the energy which leads to increase the stability. Actually, when atoms of are far separated,they do not have any force of interaction(attraction or repulsion).As they come closest that different forces operate.

1.The nucleus of one atom is attracted towards the electrons of other end and vice versa. Energy is released in a attraction.

2. The nucleus of atom as well as the electrons repel each other.Energy is needed to overcome the force of repulsion.


Formation of hydrogen(H2) molecule


If the magnitude of attractive forces is more than the repulsive forces, then stable molecule will be formed. However if the repulsive forces are more than attractive forces, then the atoms will not combine with each other.

 In the light of above discussion, let us consider the combination between the atoms of hydrogen Ha
and Hb .if Eand Eb are the respective electrons, then the attraction and the force of repulsion may be shown as

Although the number of new attractive and repulsive forces is the same, but the magnitude of attractive forces is more. Thus, when two hydrogen atoms approach each other the overall potential energy of system decreases. Therefore, a stable molecules of hydrogen gets formed.

The potential energy changes which takes place in the formation of hydrogen molecule may be also shown graphically as follow

When the two hydrogen atoms are far separated (at infinite distance) the potential energy is zero. As they start coming closer to each other from infinite distance, energy lost correspondingly. Ultimately, a point of minimum energy is attained when the attractive and repulsive forces are balance each other. At this stage, the hydrogen atoms are bonded together to form a molecule of hydrogen. The distance between the centre of the their nuclear is called bond length.
In case of hydrogen molecule, H-H bond is74 pm. It may be kept in mind start that  two hydrogen atom cannot be brought closer then 74pm.

If it happens then the repulsive forces become more than attractive forces. As a result, potential energy will increase as shown by line. The total decrease in the potential energy when one mole of bond of particular type are formed between the atoms in gaseous state is called bond energy. For example the bond energy of hydrogen hydrogen is 433 kilo joule per mole. It may be noted that the same energy is needed to break the molecules into the atoms is known as bond dissociation energy Thus,bond dissociation energy of hydrogen is also 433 kilo joule per mole.

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