Metallic and ionic crystals:
a. Similarities:
1. Both ionic and metallic crystals have electrostatic forces of attraction.
2. In ionic crystals, these are between the oppositely charged ions. In metals, these are among the valence electrons (negatively charged) and the positively charged metal ions (kernels).
3. This is the reason why both metallic and ionic crystals have high melting points.
4. In both the cases, the bond is non-directional.
b. Differences:
1. In ionic crystals, the ions are not free to move. Hence, they cannot conduct electricity in the solid state. They can do so only in the molten state or in aqueous solution.
2. In metals, the valence electrons are free to flow. Hence, they can conduct electricity in the solid state.
3. Ionic bond is strong due to electrostatic forces of attraction.
4. Metallic bond may be weak or strong depending upon the number of valence electrons and the size of the kernels.
5. On application of shearing force, ionic crystals undergo distortion and fracture in the crystal structure while on application of shearing force, metals are not fractured, but rather the layers of metallic ions slide on one another.
6. Metallic solids are malleable (hammered into sheets) and ductile (drawn into thin wires) whereas ionic solids are neither malleable nor ductile.
ii. Ionic solids are hard and brittle:
a. In ionic crystalline solids, constituent particles are positively charged cations and negatively charged anions placed at alternate lattice points.
b. The ions are held by strong coulombic electrostatic forces of attraction compensating opposite forces. Hence, they are hard.
c. Since there are no free electrons, they are not malleable and on applying a shearing force, ionic crystals break into small units. Hence, they are brittle.
iii. Solid ice is lighter than water:
a. The volume of a single water molecule (as obtained from the knowledge of the atomic radii of hydrogen and oxygen atoms) is found to be equal to 15\(\times\) 10-24 cm3.
b. If it is assumed that all the water molecules in liquid state are closely packed, the total volume of 1 mole (i.e., 18 g) of water may be obtained by the multiplication of volume of one molecule of water with the Avogadro’s number, 6.022\(\times
\)1023.
Hence, the volume of 1 mole of water = 15\(\times\)10-24 cm3 \(\times\)6.022\(\times\)1023 = 9 cm3.
c. It is found that the volume of 1 mole of water with density 1 g mL-1 is equal to 18 cm3 . It is evident that out of the total 18 cm3 (the molar volume of water), the actual volume occupied by water molecules is about 9 cm3 and the remaining half (i.e., 9 cm3 ) of the total volume is empty.
d. According to X-ray studies, the structure of solid ice is almost identical to that of liquid water. Intermolecular hydrogen bonding present in ice forms a hexagonal three dimensional crystal structure. About one half of the total space remains unoccupied. When ice melts, some of the hydrogen bonds are broken. Water molecules occupy some of the empty spaces.
e. Thus, the hexagonal crystalline structure of solid ice collapses and the liquid water molecules are more closely spaced. Due to this, the density of liquid water is higher than that of solid ice. Since solid ice is lighter than liquid water, ice floats on water.
Note: Usually the density of solid state is more than that of liquid state, but water is an exception.
