Physical Metallurgy & Structure of Solids – Ionic Solids

Solid materials are made up of large collection of atoms where the individual atoms are bonded together. The type of bonding which exists between atoms is different for all material.

An atom with a completely filled outer electron shell has a high degree of stability. The atoms of other elements with incomplete outer shells attempts, by combination with other atoms, to satisfy this condition for stability. The principal types of inter atomic bonds which may be formed are the ionic, covalent, metallic, Van Der Wall’s or molecular bonds.

The solids are also classified as per the bond structure found in them. They are IonicMetallicCovalent and Molecular.

Ionic Solids

Ionic Bond.  An ionic bond is formed when the outer most orbit electrons from one atom are transferred to another atom as a result of which each atom acquires 8 outermost electrons (octet configuration). In this process, the atom which loses an electron, develops a positive charge and the atom that gains an electron develops a negative charge. As a result of these charges, an electrostatic force of attraction develops between the atoms, which bond the atoms together.

The above images schematically show the process that takes place during the formation of an ionic bond between sodium and chlorine atoms. Note that sodium has one valence electron that it would like to give up so that it would become stable with a full outer shell of eight. Also note that chlorine has seven valence electrons and it would like to gain an electron in order to have a full shell of eight. The transfer of the electron causes the previously neutral sodium atom to become a positively charged ion (cation), and the previously neutral chlorine atom to become a negatively charged ion (anion). The attraction for the cation and the anion is called the ionic bond.

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Ionic Solids. This solid consist of alternating arrangements of positive and negative ions in a uniform pattern throughout the crystal lattice. Ionic solids are generally rigid and crystalline since the ions present are tightly held by electrostatic force of attraction.

Properties of Ionic Solid.  Ionic solids are bad conductor of electricity in the solid state while good conductor of electricity at the liquid or molecular state. Ionic solids generally have high melting and boiling points. In ionic solids the electrostatic force of attraction holds the charged atoms very strongly. Hence large amount of energy has to be supplied for breaking the bond and so the boiling point of ionic solid is found to be high. Ionic solids are freely soluble in polar solvent like water or alcohol. Example of ionic solids are NaCl,   BaCl, AlCl3 , etc.

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