The document discusses the historical development of atomic models from Thomson's plum pudding model to Rutherford's nuclear model to Bohr's planetary model. It describes key experiments and contributions from Thomson, Rutherford, and Bohr that led to advances in understanding the structure of the atom, including Thomson discovering the electron, Rutherford showing the nucleus, and Bohr incorporating quantum theory. The models progressed from electrons distributed in a positive cloud to orbiting a dense nucleus to orbiting in discrete shells.

2. Prepared By-
Meeran Ali Ahmad
Class IX-A
Roll No-08

3. What Is an ATOM?
The smallest unit of
matter that retains
the identity of the
substance is known
as atom.

4. Sir Joseph John Thomson

5. Thomson’s model of atom
(Plum pudding model)
 The plum pudding model of the atom by J. J.
Thomson, who discovered the electron in 1897, was
proposed in 1904 before the discovery of the atomic
nucleus. In this model, the atom is composed of
electrons (which Thomson still called "corpuscles",
though G. J. Stoney had proposed that atoms of
electricity be called electrons in 1894 surrounded by a
soup of positive charge to balance the electrons'
negative charges, like negatively-charged "plums"
surrounded by positively-charged "pudding". The
electrons (as we know them today) were thought to
be positioned throughout the atom, but with many
structures possible for positioning multiple electrons,
particularly rotating rings of electrons (see below).
Instead of a soup, the atom was also sometimes said
to have had a "cloud" of positive charge.

6. • In this model, the electrons were free to rotate within the blob
or cloud of positive substance. These orbits were stabilized in
the model by the fact that when an electron moved farther
from the center of the positive cloud, it felt a larger net
positive inward force, because there was more material of
opposite charge, inside its orbit ( Gauss's law). In Thomson's
model, electrons were free to rotate in rings which were
further stabilized by interactions between the electrons, and
spectra were to be accounted for by energy differences of
different ring orbits. Thomson attempted to make his model
account for some of the major spectral lines known for some
elements, but was not notably successful at this.
Still, Thomson's model (along with a similar Saturnian ring
model for atomic electrons, also put forward in 1904 by
Nagaoka after James Clerk Maxwell's model of Saturn's
rings), were earlier harbingers of the later and more
successful solar-system-like Bohr model of the atom.

7. Contributions-
 Thomson devised the famous plum pudding model of
the atom, in which electrons were compared to negative
plums embedded in a positively charged pudding. The
idea was wrong, and his successor at Cambridge,
Ernest Rutherford, was soon to develop the nuclear
model of the atom.
 Thomson investigated positive rays, which consist of
ionized atoms, beginning in 1906. He was able to use a
combination of electric and magnetic fields to separate
different charged atoms of elements on the basis of their
charge/mass ratios.
 Thomson was a great advocate of pure research, in
contrast to applied research, declaring: Research in
applied science leads to reforms, research in pure
science leads to revolutions, and revolutions, whether
political or industrial, are exceedingly profitable things if
you are on the winning side.

8. Ernst Rutherford

9. Ernst Rutherford’s model of
atom
 The Rutherford model or planetary model is a
model of the atom devised by Ernest Rutherford.
Rutherford directed the famous Geiger-Marsden
experiment in 1909, which suggested on
Rutherford's 1911 analysis that the so-called
"plum pudding model" of J. J. Thomson of the
atom was incorrect. Rutherford's new model for
the atom, based on the experimental results, had
the new features of a relatively high central
charge concentrated into a very small volume in
comparison to the rest of the atom and
containing the bulk of the atomic mass (the
nucleus of the atom).

10. • Rutherford's model did not make any new headway in
explaining the electron-structure of the atom; in this regard
Rutherford merely mentioned earlier atomic models in
which a number of tiny electrons circled the nucleus like
planets around the sun, or a ring around a planet (such as
Saturn). However, by implication, Rutherford's
concentration of most of the atom's mass into a very small
core made a planetary model an even more likely
metaphor than before, as such a core would contain most
of the atom's mass, in an analogous way to the Sun
containing most of the solar system's mass.

11. Contributions-
 After Rutherford's discovery, scientists started to realize
that the atom is not ultimately a single particle, but is made
up of far smaller subatomic particles. Following research
was done to figure out the exact atomic structure which led
to Rutherford’s gold foil experiment. They eventually
discovered that atoms have a positively-charged nucleus
(with an exact atomic number of charges) in the
center, with a radius of about 1.2 x 10−15 meters x [Atomic
Mass Number]1/3. Since electrons were found to be even
smaller, this meant that the atom consists of mostly empty
space.
 Later on, scientists found the expected number of
electrons (the same as the atomic number) in an atom by
using X-rays. When an X-ray passes through an
atom, some of it is scattered, while the rest passes through
the atom. Since the X-ray loses its intensity primarily due
to scattering at electrons, by noting the rate of decrease in
X-ray intensity, the number of electrons contained in an
atom can be accurately estimated.

12. Neil Bohr

13. Bohr’s model of an atom
 In atomic physics, the Bohr model, introduced by
Neil Bohr in 1913, depicts the atom as a
small, positively charged nucleus surrounded by
electrons that travel in circular orbits around the
nucleus—similar in structure to the solar system, but
with electrostatic forces providing attraction, rather
than gravity. This was an improvement on the earlier
cubic model (1902), the plum-pudding model
(1904), the Saturnian model (1904), and the
Rutherford model (1911). Since the Bohr model is a
quantum-physics–based modification of the
Rutherford model, many sources combine the
two, referring to the Rutherford–Bohr model.

14.  The Bohr model is a primitive model of the hydrogen atom. As a
theory, it can be derived as a first-order approximation of the
hydrogen atom using the broader and much more accurate quantum
mechanics, and thus may be considered to be an obsolete scientific
theory. However, because of its simplicity, and its correct results for
selected systems (see below for application), the Bohr model is still
commonly taught to introduce students to quantum mechanics,
before moving on to the more accurate but more complex valence
shell atom. A related model was originally proposed by Arthur Erich
Haas in 1910, but was rejected. The quantum theory of the period
between Planck's discovery of the quantum (1900) and the advent of a
full-blown quantum mechanics (1925) is often referred to as the old
quantum theory.

15. Contribution-
 The Bohr model of the atom, the theory that electrons
travel in discrete orbits around the atom's nucleus.
 The shell model of the atom, where the chemical
properties of an element are determined by the electrons
in the outermost orbit.
 The correspondence principle, the basic tool of Old
quantum theory.
 The liquid drop model of the atomic nucleus.
 Identified the isotope of uranium that was responsible for
slow-neutron fission – 235U.
 Much work on the Copenhagen interpretation of quantum
mechanics.
 The principle of complementarity: that items could be
separately analyzed as having several contradictory
properties.

16. Conclusion
 Chemistry is the science of atomic matter,
especially its chemical reactions, but also
including its properties, structure, composition,
behavior, and changes as they relate the
chemical reactions. Chemistry is centrally
concerned with atoms and their interactions
with other atoms, and particularly with the
properties of chemical bonds. Chemistry is
used by us in our daily lives though we do not
realise. Chemistry is also known as ―THE
CENTRAL SCIENCE‖.
 It is difficult to imagine your life without the
knowledge of chemistry