atoms. The energy so released is called as bond enthalpy , which is corresponding to minimum in the curve depicted in Fig. . .
Conversely, . kJ of energy is required to dissociate one mole of H molecule. H (g) + . kJ mol – → H(g) + H(g) .
. Orbital Overlap Concept In the formation of hydrogen molecule, there is a minimum energy state when two hydrogen atoms are so near that their atomic orbitals undergo partial interpenetration. This partial merging of atomic orbitals is called overlapping of atomic orbitals which results in the pairing of electrons. The extent of overlap decides the strength of a covalent bond.
In general, greater the overlap the stronger is the bond formed between two atoms. Therefore, according to orbital overlap concept, the formation of a covalent bond between two atoms results by pairing of electrons present in the valence shell having opposite spins. Fig. .
The potential energy curve for the formation of H molecule as a function of internuclear distance of the H atoms. The minimum in the curve corresponds to the most stable state of H . . .
Directional Properties of Bonds As we have already seen, the covalent bond is formed by overlapping of atomic orbitals. The molecule of hydrogen is formed due to the overlap of s -orbitals of two H atoms. In case of polyatomic molecules like CH , NH and H O, the geometry of the molecules is also important in addition to the bond formation. For example why is it so that CH molecule has tetrahedral shape and HCH bond angles are .
°? Why is the shape of NH molecule pyramidal ? The valence bond theory explains the shape, the formation and directional properties of bonds in polyatomic molecules like CH , NH and H O, etc. in terms of overlap and hybridisation of atomic orbitals.
. . Overlapping of Atomic Orbitals When orbitals of two atoms come close to form bond, their overlap may be positive, negative or zero depending upon the