LIQUIDS AND SOLIDS
The attractive and repulsive force between the
atoms in the molecule are called as
London forces exist in nonpolar molecules.
These forces result from temporary charge
imbalances. The temporary charges exist because
the electrons in a molecule or ion move randomly
in the structure. The nucleus of one atom attracts
electrons form the neighboring atom. At the same
time, the electrons in one particle repel the
electrons in the neighbor and create a short lived
These temporary charges in one molecule or atom
attract opposite charges in nearby molecules or
atoms. A local slight positive charge d+ in one
molecule will be attracted to a temporary slight d-
negative charge in a neighboring molecule.
Dipole-dipole interactions Dipole-dipole interactions
exist between molecules that are polar. This requires the
presence of polar bonds and a un symmetric molecule.
These molecules have a permanent separation of positive
and negative charge. In the illustration the H end of HCl is
permanently slightly positive charge. The Cl end of HCl
has a permanent slight negative charge. the "H" in one
molecule is attracted to the "Cl" in a neighbor. The
intermolecular force is weak compared to a covalent bond.
But this dipole-dipole interaction is one of the stronger
The polar molecules, such as water molecules, have a weak,
partial negative charge at one region of the molecule, and a
partial positive charge where the hydrogen atoms are.
Thus when water molecules are close together, their
positive and negative regions are attracted to the
oppositely-charged regions of nearby molecules. This force
of attraction, is called a hydrogen bond. Each water
molecule is hydrogen bonded to four others.
Viscosity is a quantity that describes a fluid's
resistance to flow.
The higher the viscosity of a liquid, the more
slowly it flows; hydrogen bonded liquids typically
have high viscosities. Viscosity usually decreases
with increasing temperatures.
The cohesive forces between molecules down into
a liquid are shared with all neighboring atoms.
Those on the surface have no neighboring atoms
above, and exhibit stronger attractive forces upon
their nearest neighbors on the surface. This
cause an imbalance of the intermolecular forces
at the surface which causes surface tension.
STRUCTURES OF SOLIDS
The ordered arrangement of atoms, molecules or
ions in a crystalline solid means that we can
describe a crystal as being constructed by the
repetition of a simple structural unit.
The crystal structure of a material or the
arrangement of atoms in a crystal can be
described in terms of its unit cell.
Spheres can pack in close-packed and open (non
In the cubic crystal system, there are, besides the
close-packed structure (face-centered cubic) two
important packings;the simple cubic structure
and body-centered cubic structure.
COUNTING THE NUMBER OF ATOMS IN A
The atom in a unit cell are counted by determining
what fraction of each atom resides within the cell.
The number of atoms in a unit cell is counted by
noting how they are shared between neighboring
cells.An atom at the center of a cell belongs entirely to
that cell. For an fcc structure each of the eight corner
atoms is shared by eight cells, so overall they
contribute 8x 1/8=1atom to the cell.
Each atom at the center of each of the six faces
contributes ½ an atom so jointly they contribute
Total number of atoms in a fcc unit cell is 1+3=4and
the mass of the unit cell is 4 times the mass of one
Atom Location Fraction Inside Unit Cell
Anywhere else 1
The atomic radius of copper is 128pm,and the
density of copper is 8.93g/cm³. Is the copper metal
BAND THEORY OF SOLIDS
In insulators the electrons in the valence band
are separated by a large gap from the conduction
band, in conductors like metals the valence band
overlaps the conduction band, and in
semiconductors there is a small enough gap
between the valence and conduction bands that
thermal or other excitations can bridge the gap.
With such a small gap, the presence of a small
percentage of a doping material can increase
DOPING OF SEMICONDUCTORS
The addition of a small percentage of foreign
atoms in the regular crystal lattice of silicon or
germanium produces dramatic changes in their
electrical properties, producing n-type and p-type
Impurity atom with 5 valence electrons produce
n-type semiconductors by contributing extra
Impurity atoms with 3 valence electrons produce
p-type semiconductors by producing a “hole" or
An impurity of valence
five elements is added
to a valence-four
semiconductor in order
to increase the number
of free (in this case
An N-type semiconductor (N for Negative) is
obtained by carrying out doping, that is, by
adding an impurity of valence-five elements to a
valence-four semiconductor in order to increase
the number of free (in this case negative) charge
When the doping material is added, it donates
weakly-bound outer electrons to the
semiconductor atoms. This type of doping agent
is also known as donor material since it gives
away some of its electrons.
Consider the case of Si atom.Si atoms have four valence
electrons, each of which is covalently bonded with one of
four adjacent Si atoms. If an atom with five valence
electrons, such as those from group 15 (eg. phosphorus,
arsenic, or antimony), is incorporated into the crystal
lattice in place of a Si atom, then that atom will have four
covalent bonds and one un bonded electron. This extra
electron is only weakly bound to the atom and can easily be
excited into the conduction band. At normal temperatures,
virtually all such electrons are excited into the conduction
P-type semiconductor (P for Positive) is
obtained by carrying out doping, in order to
increase the number of free (in this case positive)
charge carriers. When the doping material is
added, it takes away (accepts) weakly-bound
outer electrons from the semiconductor atoms.
This type of doping agent is also known as
acceptor material and the semiconductor atoms
that have lost an electron are known as holes.
The purpose of P-type doping is to create an
abundance of holes. In the case of silicon, a
trivalent atom (typically from group IIIA of the
periodic table, such as boron or aluminium) is
substituted into the crystal lattice. The result is
that one electron is missing from one of the four
Thus the dopant atom can accept an electron
from a neighboring atoms' covalent bond to
complete the fourth bond. Such dopants are
called acceptors. The dopant atom accepts an
electron, causing the loss of half of one bond from
the neighboring atom and resulting in the
formation of a "hole".
A phase change may be written as a chemical
reaction. The transition from liquid water to
steam, for example, may be written as
H2 (l) H2 (g)
The equilibrium constant for this reaction (the
vaporization reaction) is
K = Pw
where Pw is the partial pressure of the water in
the gas phase when the reaction is at
equilibrium. This pressure is often called the
vapor pressure. The vapor pressure is literally
the partial pressure of the compound in the gas.
The boiling point corresponds to the temperature
at which the vapor pressure of the liquid equals
the atmospheric pressure.
If the liquid is open to the atmosphere, it is not
possible to sustain a pressure greater than the
atmospheric pressure, because the vapor will
simply expand until its pressure equals that of
The temperature at which the vapor pressure
exactly equals one atm is called the normal
In a closed container vapor is formed as the
molecules leave the surface of the liquid. As the
number of molecules in the vapor phase
increases, more of them strike the surface of the
liquid. Eventually the number of molecules
returning to the liquid matches the number
escaping. The liquid is in equilibrium with the
The van't Hoff equation provides a relationship between an
equilibrium constant and temperature.
ln K = - ΔHvap /R T + ΔSvap/R
For this reaction, the equilibrium constant is simply the vapor
pressure, P, which when substituted into the above equation
yields the Claussius-Clapeyron equation.
ln P = - ΔHvap /R T + ΔSvap/R
The normal boiling point, Tbpo, corresponds to the temperature at
which both the reactant and the product are in the standard
state. A pure liquid under 1 atm pressure is in the standard state.
A pure gas at 1 atm pressure is also in the standard state. Thus
in the standard state P = 1 atm. This relation allows the
Claussius-Clapeyron equation to be rewritten as
ln P = - ΔHvap / R (1/T – 1/Tbp⁰)
Tbp⁰ = - ΔHvap / ΔSvap
A phase diagram summarizes the pressures and
temperatures at which each phase is most stable.
The phase boundaries show the conditions under
which two phases can coexist in equilibrium with
each other. Three phases coexist at equilibrium at
the triple bond. A substance cannot be converted
to a liquid by the application of pressure if the
temperature is above
critical temperature of
10.34, 10.42,10.44, 10.66,10.72