Class 12th

1.30 Knowing the density of an unknown metal and the dimension of the unit cell, the atomics mass of the metal can be determined.

Let, Atomic mass of element

(M)=d* a³* NA x Z

Where, d = density

• a³= volume of the unit cell
• NA = Avogadro's number
• Z= number of atoms present in one unit cell.
• Density of the unit cell= Mass of the unit cell / Volume of the unit cell
• d= zm / a³

1.28 Coordination number is the number of the nearest neighbors with which a given atom is in contact. In an 
ionic crystal, the coordination number of an ion refers to the number of oppositely charged ions that 
surround that ion.
The coordination number of atoms in a
(a) Cubic close-packed structure is 12.
(b) Body-centred cubic structure is 8

1.27 The different solids are classified below:
a) Ionic solids: Ammonium phosphate (NH4)3PO4), LiBr
b) Metallic solid: Brass, Rb
c) Molecular solids: Tetraphosphorous decaoxide (P4O10), Iodine (I2), P4
d) Network (covalent) solids: Graphite, SiC, Si
e) Amorphous solid: Plastics

1.26 Quartzis crystalline solid with long range order and glass is amorphous solid (or pseudo solid or super cooled liquid) with short range order and has a tendency to flow. When quartzis heated, it can be converted into glass.

1.25 Amorphous solids have short-range order with irregular shapes of constituent particles. They have isotropic nature and melt over a range of temperatures. They do not have a definite enthalpy of fusion. Examples of amorphous solids are glass, rubber, plastic, etc.

1.24 Ferromagnetic substances would make better permanent magnets because when the ferromagnetic substance is placed in a magnetic field, all domains get oriented in the direction of magnetic field and strong a magnetic effect is produced.

1.23 An n-type semiconductor conducts because of the presence of extra electrons. Therefore, a group 14 element can be converted to n-type semiconductor by doping it with a group 15 element.

1.21 Two or more cations of lower valency are replaced by a cation of higher valency to maintain electrical neutrality. Hence some cation vacancies are created. For example : In an ionic solid 'NaCl', is impurity of Sr2+ is added (as SrCl2) then two Na+ ions left thus lattice sited. To maintain electrical neutrality one lattice site is occupied by Sr2+ ion while other lattice site will remain vacant.