Noble gases have 8 electrons in their outermost shell (complete octet) and outermost configuration is ns2p6. Every atom has a tendency to complete its octet by losing or gaining or by sharing electrons.

Exceptions of Octet Rule

(a)  Electron-Deficient Compounds: Compounds having less than 8 electrons in the outermost shell of the central atom.

(b) Expansion of Octet: The compounds having more than 8 electrons in the outermost shell of the central atom.

(c)  Transition Metal Ions: Transition metal ions have 9 to 18 electrons in their outermost shell.

(d) H and Li atoms attain configuration like He by gaining and losing one electron, respectively.

(e) Odd electron Bond: Compounds having unpaired electrons are also exceptions to the octet.

Solution:  Ti-3 has 9 electrons in its outermost shell.

Example 2: Which of the following compounds has the central atom with incomplete octet?

Example 3: An element forms cation by losing electrons. The configuration of the cation may be like that of :

      (a) inert gas                   (b) pseudo inert gas

      (c) inert pair                  (d) all above     

Solution: Inert gas –.8 electron in the outermost shell. Pseudo inert gas – 18 electron in the outermost shell.

Inert Pair Configuration

The heavy elements of IIIA, IV A AND V A groups form cations having charge equal to group number and two less than group number, e.g

In these elements due to the greater penetration power of s orbitals, the atom generally does not lose e- present in ns orbital, so only np electrons are lost by the atom. This effect is called “inert pair effect”


(a) Atoms get established by ion formation information of an ionic bond. One atom forms an action by losing electrons and other forms anion by accepting electrons.

(b) The octets of the atoms are completed by transfer of electrons.

(c)  Cations and anions are bonded by the electrostatic force of attraction in an ionic bond.

(d) Ionic bond is formed between metals (cations) and nonmetals (anions).

(e)  During the formation of an ionic bond, the cation can attain one of the following configurations

(1) Inert Configuration: I A and II A group metals (octet configuration

inert configuration.

(2) Pseudo Inert Configuration: Metals of I B and II B group –

f) Energies involved in ionic bond formation:

(1)Sublimation Energy – The energy required to convert solid metal to a gaseous state.

(2)  Ionisation Energy – The energy required to remove an

      an electron from the outermost shell of metal in a gaseous state.

(3)  Lattice Energy :

(i)   The energy released during the formation of an ionic bond.

(ii)  Cations and anions form crystal lattice of ionic crystal in space by the electrostatic force of attraction.

(4) Ionic compound is formed when the energy required (Sublimation energy + Ionisation energy + Dissociation energy) is less than the energy released (Electron affinity + Lattice energy) So total energy change

Factors Affecting the Formation of Ionic Bond

(a) The sublimation energy and ionisation potential of metal should be minimal so that it may easily form a cation.

(b) The dissociation energy and electron affinity of a nonmetal should be high so that it may easily form an anion.

(c)  There should be a strong electrostatic force of attraction between cation and anion.

(d) Metals have low ionisation potential and nonmetals have high electron affinity, so ionic bond is formed between metals and nonmetals.

Characteristic of Ionic Compounds

(a) Solid and Crystalline Structure :

      Ionic compounds do not show the molecular structure.

      Ionic compounds have definite crystal structures.

      CsCl has a body-centred cubic (BBC) structure.

(b) Melting and Boiling Point :

      Ionic compounds have high melting and boiling points.

(c) Hardness :

      Ionic compounds are solid with brittle nature.

(d) Conductivity :

      Ions are not free in solid-state so ionic compounds are bad conductors of electric current in solid-state but in solution and fused state electric current passes through and the ionic compounds become a good conductor of electricity.

(e) Solubility of Ionic Compounds :

      Ionic compounds are soluble in solvents which have a high dielectric constant like water.

(f) Solvation of Hydration Energy :

      The energy released by the attraction of ions and solvent molecules are called solvation energy and when the solvent is water this is called hydration energy.

Solubility in water = Lattice energy < Hydration energy