HISTORY OF
CONCEPT OF THE ATOM AND ITS STRUCTURE:
The word "Atom" has came from the Greek word "atomos"
.Atomos means indivisible . Scientist
Dalton originated this concept but it is obselete after invention of
electron, proton and neutron particles. It is observe that when any metal is
heated it emits electrons.
Dalton-Atomic-Model |
Gradually after many scientific observation it is proved that every matter have electrons
in their atom. Usually an atom is non-ionic therefore it have equal amount of
negative and positive ion .Then the main question arises among the scientist
that how these ions are placed inside the atom. First model to define that how
ions are placed inside the atom is Thomson`s Plum pudding model.
J.J.Thomson |
THOMSON`S
PLUM PUDDING MODEL:
- 8
According
to him an Atom is 10 cm diameter positively charged sphere where
positive charge is equally distributed. The electrons are embedded within this
sphere like raisin within the pudding, among the positively charged cloud of
sphere.
Plum-Pudding-Model J.J.Thomsom |
RUTHERFORD
MODEL:
Ernest-Rutherford |
Experiment:
In 1911
Rutherford obserb the deflection of
alpha(a) particles
on metal plate. Alpha particles is similar to positively charged helium atom.
Alpha particles are emitted from the nucleus of radioactive subtance.Alpha
particles are very tiny but its mass is greater than the electrons. When
Rutherford hit a metal plate with Alpha particles ,he observed that most of the
alpha particles pass through the metal plate without cause any harm on the
plate.
1) Most
of the alpha particles deflection is very minimum.
2) Few
alpha particles is actually deflected.
3) Very
few among the few alpha particles deflected more than 90 degree.
4) Very
tiny amount of alpha particles deflected back at 180 degree from the metal
Plate.
Rutherford`s
deduction:
Rutherford
cannot explain his observation by previously known Thomson`s model. Therefore
he proposed another new model of atom to explain his observation.According to
his new model he propose that most of the positive charge or mass of a matter
present in a very small central area which is call the nucleus.The electrons
are present around the central nucleus.
1) The most
of the positively charged alpha particles
penetrate the atom on its surrounding area other than the nucleus and they
feel very low repelling force therefore they have minimum
deflection.
2) When the alpha
particles goes through nearer part of the
nucleus they feel much higher repelling force and deflected
highly.
3) When the
alpha particle go directly to the nucleus, they feel huge repulsive
force and reflected back to their inital position.
-12
It is
proven after calculation that in the third case the alpha particles must be
within 10 cm nearer to the central
nucleus
-12
therefore
diameter of the central nucleus must not higher than the 10 cm. Again it is known according the gas
law that
- 8
the
diameter of nucleus is 10 cm . So the
most of the part of the atom is empty.
According
to Rutherford model electrons are within the atom placed in circular area
around the nucleus. The electrons are not static because negatively charged
electrons are attracted toward positively charged nucleus which cannot be possible (if the electrons
are goes towards the nucleus it will fuse together and all matter will be
destroyed). Therefore it is assume that all the electrons are move in a
circular orbit around the central nucleus and their attraction force is provide
its centripetal force.This model is similar to the Sun, Planets and orbits (Sun
is like a nucleus in atom and planets
are similar like electrons which are circulate around the nucleus or sun in
their orbit). Therefore it is also known as Rutherford planetary atom
model.
Planetary-Atom-Model-Rutherford |
Defects of
Rutherford`s Model:
According
to the electromagnetic law ionized moving paricles must release its energy in
form of electromagnetic radiation.Therefore moving electrons around the nucleus
must continuously release its energy. The radius and speed of electron`s
circular orbit are gradually decrease . Therefore the electrons are gradually
come nearer to the atom`s nucleus.It
- 8
is
calculated that an electron release all its energy within 10 seconds and fall on its nucleus. This
defects of unstable atom is corrected by the Rutherford`s student Niels Bohr in
the year 1913.
NIELS BOHR
ATOMIC MODEL:
Niels Bohr |
In his
model, Niels Bohr applied Plank and Einstein`s quantum law instead of
electromagnetic and mechanics laws. His model is build on the three basic
foundation , which are given below:-
1) Electrons
cannot circulate around the nucleus on any orbit.They must circulate along a fixed orbit, which paths are defined
by specific quantum law. The specific orbit which have fully multiplied by
electron`s angular momentum(mass* Velocity) = h/2p (Here h = Plank`s constant) are known permissible orbit . These permissible orbit
are known as stable or stationary or quantum orbit.
Model- Of-Niel Bohr |
A) Calculation of theory of model:
Let assume
that an electron of mass m is circulating around the
orbit having r radius and the angular velocity of the
electron is w.
Then the
momentum of electron J = Iw
2
v
Here I = mr is electron`s inertia and w = ------
r
2 v
Therefore J = mr
* ------- = m v r
r
nh
J = m v r =
--------
2p
Here n is
full number like 1,2,3 etc.They are known as principal
quantum number. Different orbit has different principal quantum number(n). The
electron`s energy increases with the increasing amount of principal quantum
number (n).
2) The
electrons not release its energy until it orbiting around the quantum orbit.
3) When
electrons of an atom are jump form higher energy orbit to lower energy orbit ,
atom release enegy. When electron jump from lower energy to higher energy ,
atom sucks energy.
During
energy radiation or suction the energy of electron on initial orbit and last
orbit are E1 and E2 respectively then the energy of photon during radiation and
suction is:-
hv = E1~ E2
( here v = frequency of radiation, h =
Planck`s constant)
QUANTUM
NUMBERS:
The
discoveries of wave Particle duality by Broglie and the uncertainty principle
by Heisenberg have completely changed the thinking about the behaviour of
electrons in an atom .The behaviour of electrons in an atom is
described by quantum theory .Each electron in an atom is characterized by a
mathematical equation (Known as wave function) Whose square value at a given
point gives the probability of finding the electron at that point .These wave
functions are obtained by solving the Appropiate Schrodinger equation.Each wave
function constitutes an orbital in an atom and depends on three quantum numbers
n,l and m as describe in the following:
Principal
Quantum Number: (Symbol: n)
Each
principal quantum number represents a major energy shell.It can have integral
values 1, 2, 3, .... .The energy shells
are represented by the symbols K, L, M, ... depending upon the values of n
equal to 1, 2, 3,..... , respectively. For one electron system, the expression
of energy is the same as that obtained in Bohr`s theory.For a multi electron
system, the energy of an orbital depends on the two quantum numbers n and l.
Azimuthal
quantum Number:(Symbol: l)
Each
Azimuthal quantum number represents a sunshell within the major energy shell
.The values of l are 0, 1, 2, ...,(n-1) and these are designated by the symbols
s, p, d, f, g, ...., respectively .This quantum number also represents the
quantized values of angular momentum of the electron in an orbital through the
expression:
h
L =V l(l+1) *
---------
2p
This
quantum number also describes the shape of an orbital which, according to the
quantum theory , is the space around the nucleus where there exits 90 to 95%
probability of finding the electron .The orbital with l = 0 is spherical , for
l = 1, the
shape is dumb-bell and for l = 2, the shape is double dumb-bell.
Atom-Otto-Hahn. |
Spin
Quantum Number:Symbol (ms):
The
electron acn also spin around its own axis . Two types of spinning, namely
clockwise and anticlockwise is
_____
possible.This
spinning produces angular momentum equal to V s(s+1)
(h/2p), where s = 1/2 the
direction of angular momentum is governed by the quantize value if z component
of angular momentum which is given as ms (h/2p) where
ms is either
equal to + 1/2 (clockwise spinning) or -1/2 (anti
clockwise spinning).
Electron Orbital Spin |
No comments:
Post a Comment