of helium which has a duplet of electrons) have a particularly stable outer shell configuration of eight (octet) electrons, ns np . The negative and positive ions are stabilized by electrostatic attraction. For example, the formation of NaCl from sodium and chlorine, according to the above scheme, can be explained as: Na → Na + + e – [Ne] 3s [Ne] Cl + e – → Cl – [Ne] s p [Ne] s p or [Ar] Na + + Cl – → NaCl or Na + Cl – Similarly the formation of CaF may be shown as: Ca → Ca + + 2e – [Ar] s [Ar] F + e – → F – [He] s p [He] s p or [Ne] Ca + + 2F – → CaF or Ca + (F – ) The bond formed, as a result of the electrostatic attraction between the positive and negative ions was termed as the electrovalent bond. The electrovalence is thus equal to the number of unit charge(s) on the ion.
Thus, calcium is assigned a positive electrovalence of two, while chlorine a negative electrovalence of one. Kössel’s postulations provide the basis for the modern concepts regarding ion-formation by electron transfer and the formation of ionic crystalline compounds. His views have proved to be of great value in the understanding and systematisation of the ionic compounds. At the same time he did recognise the fact that a large number of compounds did not fit into these concepts.
. . Octet Rule Kössel and Lewis in developed an important theory of chemical combination between atoms known as electronic theory of chemical bonding . According to this, atoms can combine either by transfer of valence electrons from one atom to another (gaining or losing) or by sharing of valence electrons in order to have an octet in their valence shells.
This is known as octet rule . . . Covalent Bond Langmuir ( ) refined the Lewis postulations by abandoning