written by malike shamel. my quizy.com

1. ATOMIC THEORY
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2. HISTORY
Ancient Greece (around 400 BC):
The concept of the atom can be traced back to ancient
Greek philosophers, notably Democritus. Democritus
proposed that matter is composed of indivisible and
indestructible particles called "atomos," meaning
indivisible.
Religious View
Isaiah 40:15
Behold, the nations are
esteemed as a drop of
the bucket, and as the
fine dust on the
scales; behold, he
taketh up the isles as
an atom.
99. Az-Zalzalah 99:10
“So whoever
does an
atom’s weight
of good will
see it”
There is no activity you may be engaged in ˹O
Prophet˺ or portion of the Quran you may be
reciting, nor any deed you ˹all˺ may be doing
except that We are a Witness over you while
doing it. Not ˹even˺ an atom’s weight is hidden
from your Lord on earth or in heaven; nor
anything smaller or larger than that, but is
˹written˺ in a perfect Record.
SURAH YUNUS 10:61

3. Theory: A theory is a carefully thought-out explanation
for observations of the natural world that has been
constructed using the scientific method, and which
brings together many facts and hypotenuse.
Postulates are statements assumed to be true without
any requirement of proof. They are built upon the
knowledge that satisfies the reader (or listener) in
terms of veracity.
Year Discoveries Scientist Name
1808 Atom John Dalton
1919 Proton Ernest Rutherford
1897 Electron J.J. Thomson
1913 Atomic Structure Neils Bohr & Rutherford
1932 Neutron James Chadwick

4. JOHN DALTON
Dalton's fascination with gases gradually led him to formally assert
that every form of matter (whether solid, liquid or gas) was also made
up of small individual particles.
The English chemist John Dalton suggested that all matter is made up of
atoms, which were indivisible and indestructible
The following are the postulates of his theory:
● Every matter is made up of atoms.
● Atoms are indivisible.
● Atom of the same alike while atom of different are different.
● Atom of the same can not combine while atom of different can
combine.
● Specific elements have only one type of atom in them.
● Each atom has its constant mass that varies from element to element.
● Atoms undergo rearrangement during a chemical reaction.
● Atoms can neither be created nor be destroyed but can be
transformed from one form
Limitations of Dalton’s Atomic
Theory
● The theory was unable to explain the
existence of isotopes.
● Nothing about the structure of the
atom was appropriately explained.
● Later, scientists discovered particles
inside the atom that proved, that the
atoms are divisible.

5. J.J Thomson Atomic Model
The English chemist Sir Joseph John Thomson put forth his model
describing the atomic structure in the early 1900s
Observations
● When a high-voltage power supply is switched on,
rays were emerging from the cathode towards the
anode. This was confirmed by the ‘Fluorescent
spots’ on the ZnS screen used. These rays were
called “Cathode Rays”.
● When an external electric field is applied, the
cathode rays get deflected toward the positive
electrode, but in the absence of an electric field,
they travel in a straight line.
● When rotor Blades are placed in the path of the
cathode rays, they seem to rotate. This proves that
the cathode rays are made up of particles of a
certain mass so that they have some energy.
● With all this evidence, Thomson concluded that
cathode rays are made of negatively charged
particles called “electrons”.

6. Conclusions
Based on conclusions from his cathode ray experiment,
Thomson described the atomic structure as a positively
charged sphere into which negatively charged electrons
were embedded.
It is commonly referred to as the “plum pudding model”
because it can be visualized as a plum pudding dish
where the pudding describes the positively charged atom
and the plum pieces describe the electrons.
Thomson’s atomic structure described atoms as
electrically neutral, i.e. the positive and the negative
charges were of equal magnitude.
Limitations of Thomson’s Atomic Structure
Thomson’s atomic model does not clearly explain
the stability of an atom. Also, further discoveries of
other subatomic particles, couldn’t be placed inside
his atomic model.

7. Rutherford Atomic Theory
Rutherford, a student of J. J. Thomson
modified the atomic structure with the
discovery of another subatomic particle
called “Nucleus”. His atomic model is based
on the Alpha ray scattering experiment.
Construction
● A very thin gold foil of 1000 atoms thick is
taken.
● Alpha rays (doubly charged Helium He2+)
were made to bombard the gold foil.
● Zn S screen is placed behind the gold foil.
Observations
● Most of the rays just went through the gold foil making
scintillations (bright spots) in the ZnS screen.
● A few rays get reflected after hitting the gold foil.
● One in 1000 rays get reflected by an angle of 180°
(retraced path) after hitting the gold foil.

8. Conclusions
● Since most rays passed through,
Rutherford concluded that most of
the space inside the atom is empty.
● Few rays get reflected because of the
repulsion of its positive with some
other positive charge inside the atom.
● 1/1000th of rays got strongly
deflected because of a very strong
positive charge in the center of the
atom. He called this strong positive
charge as “nucleus”.
● He said most of the charge and mass
of the atom resides in the Nucleus

9. Bohr’s Model of an Atom
The Bohr model of the atom was proposed by
Neil Bohr in 1915. It came into existence with
the modification of Rutherford’s model of an
atom. Rutherford’s model introduced the
nuclear model of an atom, in which he
explained that a nucleus (positively charged)
is surrounded by negatively charged
electrons.
Bohr's theory modified the atomic
structure model by explaining that
electrons move in fixed orbitals (shells)
and not anywhere in between and he also
explained that each orbit (shell) has a fixed
energy. Rutherford explained the nucleus
of an atom and Bohr modified that model
into electrons and their energy levels.

10. Postulates of Bohr’s Model of an Atom
● In an atom, electrons (negatively charged)
revolve around the positively charged nucleus in
a definite circular path called orbits or shells.
● Each orbit or shell has fixed energy and these
circular orbits are known as orbital shells.
● The energy levels are represented by an integer
(n=1, 2, 3…) known as the quantum number. This
range of quantum numbers starts from the
nucleus side with n=1 having the lowest energy
level. The orbits n=1, 2, 3, 4… are assigned as K,
L, M, N…. shells and when an electron attains the
lowest energy level, it is said to be in the ground
state.
● The electrons in an atom move from a lower
energy level to a higher energy level by gaining
the required energy and an electron moves from
a higher energy level to a lower energy level by
losing energy.

11. Limitations of Bohr’s Model
of an Atom
● Bohr’s model of an atom failed
to explain the Zeeman Effect
(effect of magnetic field on the
spectra of atoms).
● It also failed to explain the Stark
effect (effect of electric field on
the spectra of atoms).
● It violates the Heisenberg
Uncertainty Principle.
● It could not explain the spectra
obtained from larger atoms