Electrovalent (or Ionic) Bond

The sodium atom has one electron in its outermost shell. If it loses the electron from its M shell, then its L shell now becomes the outer shell, and that has a stable octet. The nucleus of this atom still has 11 protons, but the number of electrons has become 10, so there is a net positive charge, giving us a sodium cation Na+.

On the other hand, chlorine has seven electrons in its outermost shell, and it requires one more electron to complete its octet. If sodium and chlorine were to react, the electron lost by sodium could be taken up by chlorine. After gaining an electron, the chlorine atom gets a unit negative charge because its nucleus has 17 protons and 18 electrons in its K, L and M shells. This gives us a chloride anion C1–. So both these elements can have a give-and-take relation between them.

Sodium and chloride ions, being oppositely charged, attract each other and are held by strong electrostatic forces of attraction to exist as sodium chloride (NaCl). It should be noted that sodium chloride does not exist as molecules but as aggregates of oppositely charged ions.

Formation of an ionic compound, magnesium chloride:-

The compounds formed by the transfer of electrons from a metal to a non-metal are known as ionic compounds or electrovalent compounds.

PROPERTIES OF IONIC COMPOUNDS:-

Melting point and boiling points of some ionic compounds

(i) Physical nature: Ionic compounds are solids and are somewhat hard because of the strong force of attraction between the positive and negative ions. These compounds are generally brittle and break into pieces when pressure is applied.

(ii) Melting and Boiling points: Ionic compounds have high melting and boiling points (see Table 3.4). This is because a considerable amount of energy is required to break the solid inter-ionic attraction.

(iii) Solubility: Electrovalent compounds are generally soluble in water and insoluble in solvents such as kerosene, petrol, etc.

(iv) Conduction of Electricity: The conduction of electricity through a solution involves the movement of charged particles. A solution of an ionic compound in water contains ions that move to the opposite electrodes when electricity passes through the solution. Ionic compounds in the solid state do not conduct electricity because the movement of ions in the solid is not possible due to their rigid structure. But ionic compounds conduct electricity in the molten state. This is possible in the molten state since the electrostatic forces of attraction between the oppositely charged ions are overcome due to the heat. Thus, the ions move freely and conduct electricity.