Theories
about Structure of Atom
Dalton, Thomson, Rutherford, Neil Bohr
By learningcentury.blogspot.com
1) DALTON’S ATOMIC THEORY
(As a base for modern atomic concept)
HISTORY:
John Dalton’s
presented his theory about atom in 1808.
DALTON’S ATOMIC THEORY:
POSTULATES:
The
main postulates of Dalton’s Atomic Theory are as follows;
1)
Matte is composed of very small particles
called “atoms”.
2)
An atom is an indivisible particle.
3)
Atom can neither be created nor
destroyed.
4)
Atoms of particular elements are
identical in size, shape, mass and also in other properties.
5)
Atoms of different elements are different
in their properties.
6)
Atoms combine with each other in small
whole numbers ratio to form compounds.
7)
All chemical reactions are due to
combination or separation of atoms.
BASE FOR MODERN ATOMIC THEORIES:
1) The
composition of matter is always a mystery to scientists.
2) This
mystery was solved by John Dalton an English School Teacher after a series of
experiments.
3) He
concluded that all matter must be composed of tiny particles, which are like
solid balls and cannot be further divided.
4) He
called them atoms.
2) THOMSON
ATOMIC MODEL,
HISTORY:
In
1904, William Thomson put forward a theoretical description about the inner
structure of atoms, which was strongly sustained by Sir Joseph John Thomson,
who had discovered the electron in 1897.
The
existence of Protons was also the fact that atoms were neutral in charge. Since
atom is neutral i-e protons and electrons has opposite charges. As After the
discovery of subatomic particles, it was a big step put forward by scientist
about the arrangement of subatomic particles. So that Thomson also made some
representative model about Atom which is known as “plum pudding model about atom”.
PLUM PUDDING MODEL OF ATOM:
According
to Thomson;
1.
The electrons were
embedded in a constant sphere of positive charge like blueberries trapped into
a muffin.
2.
The positive matter
was thought to be a thick soup.
3.
The electrons were
somewhat mobile. As they got closer to the outer portion of the atom, the
positive charge in the region was greater than the neighbouring negative charges
and the electron would be pulled back more toward the centre region of the
atom.
3) RUTHERFORD’S ATOMIC MODEL:
HISTORY:
In 1911, Rutherford performed an experiment in order to know the arrangement of electrons and protons in an atom.
In 1911, Rutherford performed an experiment in order to know the arrangement of electrons and protons in an atom.
EXPERIMENT:
1)
Rutherford performed an experiment to
determine the internal structure of an atom in 1911.
2)
For this purpose, he used a thin
(0.00004cm) gold foil.
3)
The gold foil was surrounded by
photographic plate or Zinc sulphide (ZnS) fluorescent screen to detect the particles
emitting from the radiation.
4)
He bombarded the gold foil with
alpha-particles (20,000) from a radioactive source (Polonium metal).
OBSERVATIONS:
1)
He observed that most of the
alpha-particles (19990) passed through the gold foil without any change in direction.
2)
It showed that most of the volume
occupied by the atom is empty.
3)
Some alpha-particles (8) were deflected
through smaller angle.
4)
Only few (2) were bounced back at their
original way.
CONCLUSION:
1) Rutherford
concluded that an atom contains a positive proton i-e nucleus in the centre of
the atom.
2) So,
alpha-particles passing near this portion were repelled.
3) Because,
alpha-particles are also positive charged particles and similar charges repel
each other.
4) If
alpha-particles are closely pass near to nucleus, they deflected through large
angles.
5) Similarly,
if they don’t pass close to nucleus, they either get deflected through very
small angles or do not get deflected at all.
MAIN POINTS/ POSTULATES:
The
main points of Rutherford’s atomic model are as follows;
1)
Atom consists of positively charged
portion called nucleus which contains proton and neutron.
2)
Electrons are revolving around the
nucleus with very high speed just like the planets revolving around the sun.
3)
The electrons revolving around the
nucleus would require centripetal force. The attractive force of the nucleus on
electrons provides centripetal force to the electron.
4)
The size of nucleus is very small as
compared to the size of atom and most of the volume occupied by the atom is
empty.
5)
Atom is the neutral. As the number of
electron is numerically equal to that of protons.
6)
Nucleus is responsible for mass and
energy of the atom.
DEFECTS IN RUTHERFORD’S ATOMIC MODEL:
Rutherford’s atomic model of
an atom resembles our solar system. It has the following defects;
1) Rutherford’s
atomic model is based on the laws of motion and gravitation, which are
applicable to the neutral bodies and not on the charged bodies.
2) According to the Maxwell theory, electrons are charged bodies revolving
around the nucleus, and should emit energy continuously. The
electron will move on the circular path and ultimately fall in nucleus.
However, it does not happen.
3) If
electrons radiate energy continuously then continuous spectrum will be obtained
but actually it gives line spectrum.
4) It
does not provide explanation about the chemical properties of the elements.
4)
BOHR’S ATOMIC MODEL
HISTORY:
In
1913, Neil Bohr presented a new atomic theory to overcome the defects of
Rutherford’s atomic model.
MAIN
POSTULATES/ POINTS:
This
atomic model was based on the following assumptions;
1)
Electrons are revolving around the nucleus in
one of the fixed circular paths called shells or orbits or also called energy
level. Each orbit has a fixed energy.
2)
The energy of the electron in an orbit is
proportional to its distance from the nucleus. The farther the electron from
the nucleus, the higher will be the energy and vice-versa.
3)
As long as electrons are revolving around the
nucleus in fixed circular orbits, they don’t gain or lose energy i-e energy of
an orbit is fixed.
4)
When an electron jumps from a higher energy
orbit to the lower energy orbit, it radiates energy and when it jumps from
lower energy orbit to higher energy orbit, it absorbs energy. The energy
difference between two levels is given by,
∆E= E2-E1 = hf
Where,
h=
Plank’s constant
f=
Frequency
E1=
the lower energy orbit
E2=
the higher energy orbit
E=
the energy difference.
5)
Electrons can reside in any one of the orbits
and cannot stay between them.
6)
Angular momentum (mvr) of an electron is an
integral multiple h/2Ï€
mvr
= nh/2Ï€
Where
n= number of shells (1.2.3……….)
m= mass of electron
v= velocity of an electron
r= radius of the orbit which electron is revolving
h= Plank’s Constant its value is 6.6262x10-34
n= number of shells (1.2.3……….)
m= mass of electron
v= velocity of an electron
r= radius of the orbit which electron is revolving
h= Plank’s Constant its value is 6.6262x10-34
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