Chemistry Study Material

The structures of crystals and molecules are often being identified using x-ray diffraction studies, which are explained by Bragg’s Law. The law explains the relationship between an x-ray light shooting into and its reflection off from crystal surface. Introduction Bragg’s Law was introduced by Sir W.H. Bragg and his son Sir W.L. Bragg. The law states that when the x-ray is incident onto a  crystal  surface, its angle of incidence,  θ θ , will reflect back with a same angle of scattering,  θ θ . And, when the path difference,  d d  is equal to a whole number,  n n , of wavelength, a constructive interference will occur. Consider a single crystal with aligned planes of lattice points separated by a distance  d . Monochromatic X-rays A, B, and C are incident upon the crystal at an angle  θ . They reflect off atoms X, Y, or Z. The path difference between the ray reflected at atom X and the ray reflected at atom Y can be seen to be 2YX. From the Law of Sines we can express thi

Introduction :- . POINT DEFECTS Point defects are localized disruptions in otherwise perfect atomic or ionic arrangements in crystal structure. These imperfection may be introduced by movement of atoms or ions. i) Vacancy This is the simplest point defect. In this system, an atom is missing from its regular atomic site. It formed during solidification as a result of atomic vibrations and during recovery as a result of local rearrangement of atoms. Vacancies are also introduced during plastic deformation. ii) Interstitialcy or Self-interstitial Self-interstitial defect in a solid is obtained when an atom in a crystal occupies an interstitial site between surrounding atoms in normal atom sites. Impurities in Solid A pure metal consisting of just only one type of atom is not possible; impurity or foreign atoms will always be present, and some will exist as crystalline point defects. Solid Solution The simplest type of alloy is that of the soli

Q.If 'a' stands for the edge length of the cubic systems: simple cubic, body centred cubic and face centred cubic, then the ratio of the radii of the spheres in these systems will be respectively: A . B . C. D. Answer:Option-A Atomic packing factor  Atomic packing factor (APF) or packing efficiency indicates how closely atoms are packed in a unit cell and is given by the ratio of volume of atoms in the unit cell and volume of the unit cell FCC lattice In the FCC unit cell effective number of atoms = 8 corner atoms x (1/8) (each atom is shared by 8 unit cells) + 6 facecentered atoms x1/2 (each shared by two unit cells) = 4 Planar dencity Close-Packed Structure  FCC and hexagonal crystal structures are most highly packed with packing efficiency of 74% (APF= 0.74). Such structures can be described in terms of close-packed atomic planes.  In FCC, {111} planes are close-packed and the basal pla

Planes in a crystal are described by notations called Miller indices  Miller indices of a plane, indicated by h k l, are given by the reciprocal of the intercepts of the plane on the three axes.  The plane, which intersects X axis at 1 (one lattice parameter) and is parallel to Y and Z axes, has Miller indices h = 1/1 = 1, k = 1/ = 0, l = 1/ = 0. It is written as (hkl) = (100).  Crystal Planes To find the Miller Indices of a plane, follow these steps:  Determine the intercepts of the plane along the crystal axes  Take the reciprocals  Clear fractions  Reduce to lowest terms and enclose in brackets () Ex: Intercepts on a, b, c : ¾, ½, ¼ (h k l) = (4/3, 2, 4) = (2 3 6) Planes can also have negative intercept e.g. 1, -1/2, 1 h k l = 1 -2 1. This is denoted as ( ) 121  Family of planes {hkl}  Planes having similar indices are equivalent, e.g. faces of the cube (100), (010) and (001). This is termed as a family of planes and denoted as {100} which includes all th

1 Introduction A large majority of substances around us are solids. The distinctive features of solids are: They have a definite shape. They are rigid and hard. They have fixed volume. These characteristics can be explained on the baisi of following facts: The costituent units of solids are held very close to each other so that the packing of the costituents is very efficient. Consequently solids have high densities. Since the constiuents of solids are closely packed, it imparts rigidity and hardness to solids. The costituents of solids are held toghetes by strong forces of actraction. This results in their having define shape and fixed volume. Information regarding the nature of chemical forces in solids can be obtained by the study of the structure of solids, i.e. arrangements of atoms in space. 2 Classsification of solids Solids are classified into categories: Amorphous solids Crystalline solids The two types of solids have different characteristics.