The Solid State

Question 257

‘Crystalline solids are anisotropic in nature’. What does this statement mean?


All crystalline solids are not anisotropic. Those crystalline solids which are anisotropic have their atoms arranged and spaced in a different manner in three different planes (X, Y and Z). Therefore, the physical properties of crystalline solids such as electrical resistance or refractive index show different values when measured along different directions in the same crystals.

Example NaCl, Quartz, Ice, HCl, Iron, etc.


Question 258

Silver crystallizes in face-center cubic unit cell. Each side of this unit cell has a length of 400 pm. Calculate the radius of the silver atom. (Assume the atoms just touch each other on the diagonal across the face of the unit cell. That is each face atom is touching the four corner atoms.) 


Given, silver crystallizes in fcc unit cell 
space straight r equals space fraction numerator straight a over denominator 2 square root of 2 end fraction

Where space straight r space is space the space radius space of space the space solver space atom space and space straight a space is space the space edge space length
Now comma space edge space length space equals space 400 pm space equals space 400 space straight x space 10 to the power of negative 10 end exponent space cm

Thus comma

straight r space fraction numerator 400 space straight x space 10 to the power of negative 10 end exponent over denominator 2 space straight x space 1.414 end fraction space equals space 141.44 space straight x space 10 to the power of negative 10 end exponent cm space equals space 141.4 space pm

Thus comma space the space radius space of space the space silver space atoms space was space found space to space be space 141.4 space pm

Question 259

Define the following terms:

(i) Primitive unit cells

(ii) Schottky defect

(iii) Ferromagnetism


(i) Primitive unit cells are defined as the unit cells in which the constituent particles are present only at the corner positions.

(ii) Schottky defect: Schottky defect is basically a vacancy defect shown ionic solids. In this defect, an equal no cation and anion are missing to maintain electrical neutrality. It decreases the density of a substance. An Ionic substance containing the similar size of cations and anion show this type of defect. For example: NaCl, KCl, CsCl, AgBr, etc.

(iii) Ferromagnetism is defined as the phenomenon in which substances, such as iron, cobalt and nickel, are strongly attracted by a magnetic field. Such substances are called ferromagnetic substances.

Question 260

(a) Why does the presence of an excess of lithium make LiCl crystals pink?

(b) A solid with cubic crystal is made of two elements P and Q. Atoms of Q are at the corners of the cube and P at the body-center. What is the formula of the compound? 


(a) When crystals of LiCl is heated in presence of excess of lithium, Cl- ions from crystal diffuse on the surface and combine with ionized Li to form LiCl. The released unpaired electrons occupy the anionic sites known as F-centers. The pink colour results by excitation of these electrons when they absorb energy from visible light falling on them.

(b) It is given that the atoms of Q are present at the corners of the cube. Therefore, number of atoms of Q in one unit cell =

Also, it is also given that the atoms of P are present at the body-center.

Therefore, number of atoms of P in one unit cellequals 1 over 8 straight x 8 = 1

This means that the ratio of the number of P atoms to the number of Q atoms, P: Q = 1:1

Hence, the formula of the compound is PQ.

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