Elements can gain or lose electrons in order to attain their nearest noble gas configuration. Formation of ions (either by gaining or losing electrons) for the completion of octet helps them gain stability. In a reaction between metals and non-metals, metals generally loose electrons to complete their octet while non-metals gain electrons to complete their octet. Metals and non-metals generally react to form ionic compounds.
For example reaction between magnesium and chlorine. Magnesium atom has two electrons in its outermost shell. By losing two electrons from its M shell its L shell becomes the outermost shell which has a stable octet. The nucleus of this magnesium atom still has twelve protons but the number of electrons has decreased to ten. So, a net positive charge is developed on this magnesium atom, giving a magnesium cation Mg2+.
On the other hand, chlorine atom has seven electrons in its outermost shell. Therefore, it needs only one electron to complete its octet. It can gain this one electron from the electrons lost by magnesium atom to become magnesium ion. As two electrons are lost by magnesium atom while one chlorine atom can gain only one electron, two atoms of chlorine combine with one atom of magnesium to form magnesium chloride.
From the above example, ionic compounds can be defined as the compounds formed by the transfer of electrons between metals and non-metals. The bond formed between them is known as the ionic bond. Due to the presence of oppositely charged ions, ionic compounds are held strongly by the electrostatic force of attraction. Prominent properties of ionic compounds are:
Due to the presence of the strong force of attraction between the positive and negative ions, ionic compounds are solids and are hard to break. They generally break into pieces when pressure is applied, hence are considered brittle.
Due to the presence of electrostatic forces of attraction between ions, a large amount of energy is required to break the ionic bonds between the atoms. Thus, ionic compounds have high melting and boiling points.
Ionic compounds are generally soluble in polar solvents such as water whereas the solubility tends to decrease in non-polar solvents such as petrol, gasoline etc.
Ionic compounds do not conduct electricity in the solid state but are good conductors in a molten state. Conduction of electricity involves the flow of charge from one point to another. In the solid state, as the movement of ions is not possible, ionic compounds don’t conduct electricity. Whereas in the molten state, ionic compounds conduct electricity as electrostatic forces of attraction between the ions are overcome by the heat released.