Hack tech fact

 Packing of particles in crystal: In a crystal the constituent particles are closely packed. If maximum available space is occupied by the constituent particles, a crystal is said to be closely packed and have maximum density and stability. The close packing, however depends on the shape and size of the constituent particles in close packing. Assuming the constituent particles to be hard spheres of equal size.

 Bragg's model of X ray diffraction : According to Bragg's, a crystal which is made up of series of equally spaced atomic planes, can be treated as a transmission grating and a reflection grating as well. When X-rays are incident on a crystal face, some of the rays will pass undeflected and some other rays will penetrate into the crystal and strick the atoms in successive planes. From each of these plans the X-rays are reflected (i.e., X-rays strick with electrons in the atoms of the structural units and undergo a change in direction), in all directions. For the formation of an intense diffraction pattern in any direction the condition is that the angle of incidence should be equal the angle of reflection of the beam for the direction studied.               Bragg's equation can be applied either to the reflection or the diffraction experiment. In the case of reflection from the crystal surface, the angle ∅ stands for the angle between the cry...

Nature of diffraction patterns : The big spot in the centre of the pattern corresponds to the unscattered beam. The other spots represent the scattered beam through different characteristics angles. The spreading of X-rays from each lines of atoms in the crystal gives rise to region of various intensities. Diffraction patterns depends on the symmetry of the crystals, and helps in determining the crystal structure.

Structure of simple ionic crystals:   In ionic crystals lattice positions are occupied by positive and nagatives ions in equivalent amounts. In such crystals positive ions are surrounded by negative ions and vice-versa. Since the coulombic forces holding the oppositely charged ions together in an ionic crystal are non directional, the arrangement of ions in the crystal is largely controlled by the sizes and charges of the ions concerned. Normally, each ion is surrounded by the largest number of oppositely charged ions. The number of oppositely charged ions surrounding an ion is the co ordination number of the ion and is related to the relative sizes of the positive and nagative ions. We will discuss below the different types of ionic compounds, depending upon the relative number of positive and nagative ions present in them.

 Imperfections or Defects in crystalline solids : In an ionic crystal the particles are well orderly arranged in a regular pattern. An ideal crystal is that which has same until cell containing the same lattice points through out the whole crystal. But, such ideal crystal only exists at zero Kelvin temperature i.e., entropy of it's particle is zero. It means that there is no movement of the constituent particles at 0K. But, none of the crystals are basically ideal and it suffers certain defect at temperature above 0K. This defect may arise due to some irregularities in the arrangement of constituent particles in the crystal lattice and are called imperfections or crystal defects. These are of two types. Point defects:                 These defect are due the irregularities in the arrangement of atoms around a point or an atom in a crystalline solid. These are also called atomic defects. Line defects:            ...

Rutile structure: The radius radio is in the range 0.73 to 0.41. in rutile (TiO²) each Ti⁴+ ion is octahedrally surrounded by six O²- ions whereas each other O²- ion is surrounded by only three Ti⁴+ ions arranged at the tree corners of a plane triangle. The Co-ordination number of Ti⁴+ and O²+ ions are 6 and 3, respectively. It is not exactly a close packed structure. Ti⁴+ ions in rutile may however, be considered as forming a sufficiently distorted body centred cubic lattice.           Example  compounds  having radius ratio below 0.41 are SiO² and BeF² but these are only a few. The Co-ordination number of Si⁴+ and Be²+ is four and that of O²- is two. However, these are appreciably co-valent.

 Fluorite structure :  In CaF², the radius radio is 0.732 which gives rise to a body - centred cubic (bcc) structure. In this case each Ca²+ ion is surrounded by eight F- ions, so the co-ordination number of Ca²+ is 8. Since in an ionic compound containing different numbers of cation and anions, the cation and anion have different co-ordination numbers, it follows that the co ordination number of F- ion is four (since the number of F- ions is double the number of Ca²+ions). Thus, CaF² has 8 : 4 arrangements. In fluorite, Ca²+ ions are too small to touch each other hence, the structure is not strictly close packed structure. 

 Results of Schottky defects:  Because of missing of ions, the ionic compounds show following changes in the properties of ionic crystals.  1- The density of the crystal decreases. 2- The stability and lattice energy of crystal decreases. 3- The electrical conductivity of the crystal increases. It is because when electricity is applied, the ions move to the vacant place and this process continues in the whole crystal lattice resulting in the increase of electrical conductivity of crystals. Alkali metal halides such NaCl,KCl,KBr, CsCl etc. Normally show this defect.

Results of Frenkel defect:    1- It increase the electrical conductivity of the crystal because of the presence of vacancy in crystal lattice.  2- It decrease the stability or lattice energy of the crystal. 3- This defect does not change the density of crystal because the number of ions per unit volume remain same in the crystal. 4- It increase the dielectric constant of the crystalline solid as the similarly changed ions come closer. This defects are generally found in (i) AgCl,AgBr,Agl etc. It is because Ag+ ion being small in size occupy the interstitial sites leaving its own position in crystal lattice (ii) Further this defect is also noticed in ZnS crystal because of small size of Zn²+ ion which can fit in its own interstitial sites. (iii) Alkali metal halides don't exhibit this defect because of large size of alkali metal ions.

Position of nobel gases in the periodic table: The inert gases were not discovered at the time when Mendeleef gave his periodic table. He also could not imagine the presence of such elements devoid of chemical reactivity and thus left no place for these elements in his periodic table. As these elements are chemically inert, they should be placed in between the highly electronegativity halogens (VIIA) and highly electropositive alkali metals (IA), I.e., in the zero group. Further, it has been found that all these elements have fully filled stable electronic configuration, i.e., they have no tendency either to lose, gain or share electrons with the atoms of other elements. In other words, their valency is zero. Therefore, they have assigned zero group in the periodic table. Zero group is also numbered as group 18  in the modern periodic table. Basing upon their electronic configuration, it has been found that expect helium all other inert gases have eight electrons in their valence s...