0.676g cm^-3

Ferromagnetism: When magnetic moments are aligned in the same direction, it gives rise to ferromagnetism, e.g. iron, cobalt and nickel show ferromagnetism.
Antiferromagnetism: When magnetic moments are aligned in such a way that net magnetic moment is zero, then magnetism is called antiferromagnetism, e.g. MnO shows antiferromagnetism.

(i) The unit cell is the smallest portion of a crystal lattice which when repeated over and over again in different directions results in the entire lattice. For example, simple cubic unit cell.
(ii) The number of nearest neighbors in a packing is called coordination number.For example,in a body-centered cubic structure, the coordination number is eight.

Four

(i) Ferromagnetism: Those substances which remain magnetized even after the magnetic field is removed are called ferromagnetic substances. They have unpaired electrons, e.g. iron, cobalt, nickel, CrO_2, etc. These substances are strongly attracted by a magnetic field. The phenomenon is called Ferromagnetism.
(ii) Paramagnetism: Those substances. which have unpaired electrons are called paramagnetic substances. They are weakly attracted by a magnet, e.g. O₂, Cu²⁺, Fe²⁺ are paramagnetic in nature. The phenomenon is called Paramagnetism.
(iii) Ferrimagnetism: It is observed when the magnetic moments of the domains are aligned parallel and anti-parallel but unequal in number is called ferromagnetic substances, e.g.Fe₃O₄, CuFep.vetc. The phenomenon is called Ferrimagnetism.

\(d=\frac { Z\times M }{ { a }^{ 3 }\times { N }_{ A } } \)
\(\\ M=\frac { d\times { a }^{ 3 }\times { N }_{ A } }{ Z }\)
\( Z=\frac { 8.0g{ cm }^{ -3 }\times { \left( 250 \right) }^{ 3 }\times { 10 }^{ -30 }{ cm }^{ 3 }\times 6.023\times { 10 }^{ 23 } }{ 2 }\)
\(Z=\frac { 8\times 15.625\times 6.023 }{ 20 } \)
\(=\frac { 752.875 }{ 20 } \)
\(\\ 37.64 \ g{ \ mol }^{ -1 }\)
\(For \ bcc, \ 4r=\sqrt { 3 } a\)
\(\ r=\frac { \sqrt { 3 } a }{ 4 } =\frac { 1.732\times 250 }{ 4 } =108.25 \ pm\)

(i) Piezoelectric crystals behaves as mechanical electrical transducer. 
(ii) Piezoelectric crystals are used as pickups in record players where they produce electric signals by application of pressure. 
(iii) We generally become tense and pessimistic under stress. But we should face every situation, accept every worst situation as challenge and maintain our mental balance. It will definitely result in our performance just like in piezoelectric crystals. We can learn from these non-living crystals to be optimistic as worst situations can be converted into an opportunity.