Truly ionic crystals cannot absorb light hence they are transparent to light. Light can pass through without absorption. Most ionic salts are hard owing to the strength of the ionic bonds. They have high melting points because of their hardness. They are brittle because the atoms cannot slip past each other. In contrast metals are ductile and malleable because atoms can slip past each other. Ionic crystals are soluble in polar liquids because of their high dielectric constant. The columbic force of attraction is considerably reduced in polar liquids such as water because of the fact that:
ε 0 ε r E = D
hence E = D/(ε 0 ε r )
Water has a dielectric constant of 70 under DC condition. In DC condition polarization is contributed in major part due to the alignment of polar molecules which water molecules are. This is the main reason for its high dielectric constant. Hence when NaCl salt is put in water, because of very weak columbic attraction between Cations and Anions, the two dissociate and they give rise to very good electrolytic solution which is suitable for carrying out electrolysis. At the same time ionic salts are insoluable in covalent liquids such as benzene(C 6 H 6 ) and carbon tetrachloride(CCl 4 ) which have very low dielectric constant. Hence ionic bonds cannot be broken and ionic salts remain intact in organic solvents. The ionic salts are good IR absorbers because IR resonates with the natural frequency of the molecular ionic salt.
On one extreme we have 100% covalent compounds and covalent elements. Covalent bonds may be non-polar if the centres of charge coincide as is the case in Si and Ge and polar if the two centers are displaced as is the case in Water (H 2 O).
In water molecule two Hydrogen atoms share their two electrons with six electrons of one oxygen atom. In the process Oxygen completes its octave and Hydrogen also completes its octave and the system is at Energy Minima and hence in stable state. We say two Hydrogen atoms are covalently bonded to one Oxygen atom. In the process center of negative charge shift towards Oxygen nucleus and water molecule becomes polar.
On the other extreme we have 100% ionic bonds between Gr I and Gr VII where there is 100% transfer of electrons.
In between these two extremes we have intermediate cases where we have a mix of covalent bond and ionic bond. As we move from I-VII to II-VI to III-V the ionic bond is progressively transformed to covalent bond. In these case we have mixtures of two bands. Some electrons are exchanged and some electrons are shared. For example II-VI BaS compound is predominantly ionic and slightly covalent. As we move to III-V InP we have predominance of covalent bond.
1.10.3.3. METALLIC BONDS.
A metal is an assembly of positive ions with their valence shells completed and hence octave completed. This assembly is submerged in a sea of conducting electrons which are the outer most electrons of the atoms but which no longer belong to their host atoms but they belong to the whole crystalline lattice. The outermost lone electron is loosely held because of large orbital radius hence first ionization energy is very low. As seen in Table 1.16 for Li it is 5.4eV and as we go down the Group I the size becomes bigger hence first ionization decreases to 3.9eV for Cesium. After first ionization the metal becomes monovalent cations with very stable electron configuration. Hence second ionization is much larger about 14 times more in case of Lithium. Again because of increasing size the valence electrons are less tightly held as we go down Group I hence second ionization also decreases. It is 6 times greater than First Ionization energy in case of Cesium.