Free atomic have a random motion and prosess energies. Farther the atoms are more will be the energy of the system, giving it lesser stability. Two or more atomic unite to from a molecule because in doing so the energy of the system is lowered and thus the 'molecular' becames stable in comparison to separate atoms.
In other words, stable chemical union or bond between two (or more) atoms comes into existence only if the energy is lowered when the atoms come in close proximity.
By the term 'energy' here, we mean the sum of the Kinetic and potential energies. The Kinetic energy of atoms finds its origin in the unabated motion of the electrons.
The potential energy of a system in which atomic A and B approach each other can be due to system in which atomic A and B approach each other can be due to :
- Electron- electrons repulsions
- Nucleus - nucleus repulsions
- Nucleus - electrons attraction
Bond Formation (Valence Bond Theory)
Bond formation between atomic to give compound can be interpreted admirably in terms of the orbital theory of atomic structure.
Hitler and London believed that electron cloud of the valence orbital on one atom 'overlaps' the electron cloud of the other bonding atom to form a covalent linkage. On the country, the electrovalent bond formation involves a physical transfer of the electron and the orbital concept is not very useful for their explanation.
The theory of 'maximum overlap' of electron waves of orbitals are:
(1). The orbitals entering into combination must have only one electron.
The orbitals containing a pair of electrons are not capable of combination. In fact, half-filled orbitals on one atom have a tendency to combine with half-filled orbitals on the atom ⚛️, and the resulting orbitals acquires a pair electron of opposite spins.
(2). The atomic with valence or bonding orbital (half-filled) should approach sufficient close to one another with the axis of their orbitals in proper alignment.
The strength of a covalent bond depends upon the extent of overlapping. Greater the overlapping between the atomic orbitals stronger is the bond formed between the two atoms.
The new arrangement has lesser energy than the isolated atoms and, is therefore, more stable. The amount of energy given off or released per mole at time of overlapping of atomic orbitals to form a bond is termed as Bond Energy or Stabilization Energy. It also implies that this much energy must be put in to separate the atoms in a molecule.
Since the overlapping of atomic orbitals involves a release of energy, it must produce a stabilizing effect on the system. The merger or overlapping if the atomic orbitals half at a stage when the atomic nuclei have come close enough to exercise a repulsive force on one another, which exactly, balance the force of merger . This equilibrium distance at which is two atomic nuclei are now held is called Bond Length.
In the bond formation, greater the overlap greater is the energy set free, i.e., higher will be the bond strength. This implies that for a bond to be stronger, greatest should be the overlap which in tum shortens the distance between the nuclei.
A stronger bond has, therefore, a shorter bond length.
