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
oGeometrical shape
oSharp melting points
oSymmetry ( 3 types of elements of symmetry; plane
of symmetry, axis of symmetry & center of symmetry)
oCleavage
oIsomorphism
oPolymorphism
oAllotropy

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:
1. A no of ionic solids & metals have crystal
lattice comprising cubic unit cells.
2. 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 2θ
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.
1. 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 CO2,I2)
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