Solids state of matter

Physical states of matter

1. SOLIDS

2. Solids are the substances which are rigid, hard, have definite shape & volume. Lack the ability to flow. Why solids are rigid & have definite shape? In solids, atoms, ions & molecules are held together by strong chemical forces i.e. ionic bond, covalent bond or by intermolecular van der Waals forces/cohesive forces. They don’t translate or rotate (like liquids & gases) but only vibrate to some extent in their fixed positions.

3. TYPES OF SOLIDS Crystalline solids Amorphous solids Properties of crystalline solids Geometrical shape Sharp melting points Symmetry ( 3 types of elements of symmetry ; plane of symmetry, axis of symmetry & center of symmetry) Cleavage Isomorphism Polymorphism Allotropy

4. Transition temperature Anisotropy Habit of a crystal CRYSTAL STRUCTURE

5. Crystal Lattice The particles in crystals(atoms, ions or molecules) are located at definite positions in space. These positions are represented by points in crystal known as lattice points or lattice sites . The overall arrangement of particles in crystal is called crystal lattice/space lattice .

6. UNIT CELL: The simple basic unit or building block of the crystal lattice is called the unit cell. It has all the characteristic features of the entire crystal. Parameters of unit cell

7. A unit cell has one atom or ion at each corner of the lattice, there may be atoms/ions in faces & interior of the cell. A cell with an interior point is body centered cell & a cell with regular 3 dimensional unit cell with atoms/ions located at its corner only & does not contain any interior point is known as primitive cell . CUBIC UNIT CELLS are important for 2 reasons: A no of ionic solids & metals have crystal lattice comprising cubic unit cells. It is relatively easy to make calculations with these cells because all sides & angles are equal.

8. Three types of cubic unit cells are; Simple cubic unit cell : atoms/ions occupy only corners of cube . Body-centered cubic unit cell : has one particle at the center of cube in addition to the particles at corners. Face-centered cubic unit cell : has one particle at each of 6 faces of the cube apart from the particles at the at the corner.

9. CRYSTAL SYSTEMS

10. Illustration of seven crystal systems according to their relative axial length & interaxial angle

11. METHODS FOR ANALYSIS OF CRYSTAL STRUCTURE X-Ray Crystallography

12. The study of crystal structure with the help of x-rays is called X-Ray Crystallography. X-rays are short wavelength high energy emr produced by bombarding a metal with high energy electrons. The high energy electrons interact with the inner electrons of inner shells of atoms of metals. The collision knocks out an electron of an inner shell & an electron in a higher energy shell drops into the vacancy, emitting the excess of energy as photon of higher energy. A crystal lattice is considered to be made up of regular layers or planes of atoms equal distance apart. Since the wavelength of X-rays is comparable to the interatomic distances, Laue(1912) suggested that crystal can act as grating to X-rays.

13. Thus when a beam of X-rays is allowed to fall on a crystal, the image obtained on photographic plate would show a no. of spots. From the overall diffraction patterns produced by a crystal, we can arrive at the detailed information regarding the position of particles in the crystal. BRAGG’S EQUATION: In 1913 the father & son, W.L. Bragg & W.H. Bragg worked out a mathematical relation to determine interatomic distances from X-ray diffraction pattern. This relation is called Bragg's equation & they showed that:

14. 1. The x-ray diffracted from atoms in crystal planes obey the laws of reflection 2. The two rays reflected by successive planes will be in phase if the extra distance travelled by the second ray is an integral no of wavelengths.

15. The Rotating Crystal Method(Bragg 1913)

16. A beam of x-rays of known wavelength falls on the face of crystal mounted on a graduated turn table. The diffracted rays pass into the ionisation chamber of the recorder where they ionise the air & current flows between the chamber wall & an electrode inserted in it connected to an electrometer. The electrometer reading is proportional to intensity of x-rays. As the recorder along with crystal is rotated, the angles of max. intensity are noted on the scale.

17. The Powder Method(Debye & Scherrer,1916)

18. The crystalline material contained in capillary tube is placed in in the camera containing a film strip, the sample is rotated by means of a motor. The x-rays pass through the gap b/w the ends of the film. The powdered sample contains small crystals arranged in all orientations, some of these will reflect x-rays from each lattice plane at the same time. The reflected x-rays will make an angle 2 θ with the original direction. Hence on the photo are obtained lines of constant θ which can be calculated for different crystal planes.

19. PACKING OF ATOMS IN SOLIDS Cubic close packed structure Hexagonal close packed structure

20. 4 types of crystalline solids depending upon the types of bonds present in them. IONIC CRYSTALS: ( e.g ; NaCl & CsCl ) Lattice made of + ve & - ve ions Held together by ionic bonds (strong electrostatic attraction b/w oppositely charged ions) Brittle, hard & rigid with high melting points Shatter easily by hammering Non-conducting because ions are in fixed positions whereas in fused state may conduct electricity. Classification of crystals on the basis of bonds

21. Lattice energy of an ionic crystal (Born-Haber cycle) It may be defined as change in enthalpy that occurs when one mole of a solid crystalline substance is formed from its gaseous ions. 2.MOLECULAR CRYSTALS: ( e.g;dry CO 2 ,I 2 ) Molecules are the structural units, held together by van der Waals forces. Low melting points.

22. 3. COVALENT CRYSTALS: ( e.g;Diamond ) Atoms occupy the lattice sites & are held together by covalent bonds & produce giant molecules. Also called atomic solids. Vey hard & very high melting points. 4. METALLIC CRYSTALS: consist of atoms present at the lattice sites, held together by metallic bonds. The valence electron of metal atoms are considered to be delocalized leaving positive metal ions. The mobile electrons in the metal structure makes them excellent conductors of electricity. When stress applied metals can be deformed but the crystal doesn’t break.

23. SEMICONDUTORS Metals are good conductors of electricity while elements like silicon & germanium are non-conductors at ordinary temperature. However they exhibit appreciable conductivity upon addition of impurities such as arsenic & boron, the process is called DOPING. In silicon & germanium crystals, each atom is covalently bonded to four neighbours so that all of its four valence electrons are tied down, thus in pure state these elements are non-conductors. P-type semiconductors N-type semiconductors

24. Applications of semiconductors Solar cells Transistors Rectifiers