Other drawbacks of the octet theory • It is clear that octet rule is based upon the chemical inertness of noble gases. However, some noble gases (for example xenon and krypton) also combine with oxygen and fluorine to form a number of compounds like XeF2, KrF2, XeOF2 etc., • This theory does not account for the shape of molecules. • It does not explain the relative stability of the molecules being totally silent about the energy of a molecule. 4.2 IONIC OR ELECTROVALENT BOND From the Kössel and Lewis treatment of the formation of an ionic bond, it follows that the formation of ionic compounds would primarily depend upon: • The ease of formation of the positive and negative ions from the respective neutral atoms; • The arrangement of the positive and negative ions in the solid, that is, the lattice of the crystalline compound. The formation of a positive ion involves ionization, i.e., removal of electron(s) from the neutral atom and that of the negative ion involves the addition of electron(s) to the neutral atom. M(g) → M+(g) + e– ; Ionization enthalpy X(g) + e– → X – (g) ; Electron gain enthalpy M+(g) + X –(g) → MX(s) The electron gain enthalpy, ∆∆ ∆∆ ∆egH, is the enthalpy change (Unit 3), when a gas phase atom in its ground state gains an electron. The electron gain process may be exothermic or endothermic. The ionization, on the other hand, is always endothermic. Electron affinity, is the negative of the energy change accompanying electron gain.