J.J. Thomson discovered the electron in 1897 through experiments with cathode rays. He proposed the "plum pudding" model of the atom, where electrons were embedded in a uniform sphere of positive charge. Ernest Rutherford performed the gold foil experiment in 1909, which led him to propose the Rutherford model of the atom in 1911 - a small, dense nucleus surrounded by orbiting electrons. Neils Bohr improved on this model in 1913 by incorporating quantum theory, explaining the Rydberg formula for hydrogen spectra. In the Bohr model, electrons orbit in discrete energy levels and jump between them, emitting or absorbing photons of specific frequencies.

1. BY HIMANSHU
SAMARIYA
STD:- 9TH
DIV ;- E

2. on The Chapter
structure of an
atom

3. What is an atom ?
• An atom is an smallest partical of an element
that can take part in an chemical reaction
• Atom are very , very small in size
• Hydrogen atom is the smallest atom in all

4. Atom lock like
Structure of an atom
Structure of an atom

5. J.J.Thomeson
• Sir Joseph John "J. J." Thomson, OM, FRS1] (18 December 1856 –
30 August 1940) was a British physicist. In 1897, Thomson showed
thatcathode rays were composed of a previously unknown
negatively charged particle, and thus is credited with the
discovery and identification of theelectron. Thomson is also
credited with finding the first evidence for isotopes of a stable
(non-radioactive) element in 1913 as part of his exploration into
the composition of canal rays (positive ions) and with the
invention of the mass spectrometer. Thomson was awarded the
1906 Nobel Prize in Physics for the discovery of the electron and
for his work on the conduction of electricity in gases.

6. Discovery of an electron
•
Several scientists, such as William Prout and Norman Lockyer, had suggested that atoms
were built up from a more fundamental unit, but they envisioned this unit to be the size
particle.[5] He estimated the mass of cathode rays by measuring the heat generated when
the rays hit a thermal junction and comparing this with the magnetic deflectiof the
smallest atom, hydrogen. Thomson, in 1897, was the first to suggest that the fundamental
unit was over 1000 times smaller than an atom, suggesting the subatomic particles now
known as electrons. Thomson discovered this through his explorations on the properties
of cathode rays. Thomson made his suggestion on 30 April 1897 following his discovery
that Lenard rays could travel much further through air than expected for an atom-sized
on of the rays. His experiments suggested not only that cathode rays were over 1000
times lighter than the hydrogen atom, but also that their mass was the same whatever
type of atom they came from. He concluded that the rays were composed of very
light, negatively charged particles which were a universal building block of atoms. He
called the particles "corpuscles", but later scientists preferred the name electron which
had been suggested by George Johnstone Stoney in 1891, prior to Thomson's actual
discovery.[6]

7. • In April 1897 Thomson had only early indications that the cathode rays
could be deflected electrically (previous investigators such as Heinrich
Hertz had thought they could not be). A month after Thomson's
announcement of the corpuscle he found that he could reliably deflect
the rays by an electric field if he evacuated the discharge tube to a
very low pressure. By comparing the deflection of a beam of cathode
rays by electric and magnetic fields he obtained more robust
measurements of the mass to charge ratio that confirmed his previous
estimates.[7] This became the classic means of measuring the charge and
mass of the electron.
• Thomson believed that the corpuscles emerged from the atoms of the
trace gas inside his cathode ray tubes. He thus concluded that atoms
were divisible, and that the corpuscles were their building blocks. To
explain the overall neutral charge of the atom, he proposed that the
corpuscles were distributed in a uniform sea of positive charge; this
was the "plum pudding" model—the electrons were embedded in the
positive charge like plums in a plum pudding (although in Thomson's
model they were not stationary, but orbiting rapidly

8. Thomsons modal of an atom
• 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 order to add the electron to the atomic model. 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[1]) surrounded by a soup of
positive charge to balance the electrons' negative charges, like
negatively charged "raisins" 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.

9. •
•
With this model, Thomson abandoned his earlier "nebular atom" hypothesis in
which the atom was composed of immaterial vortices. Now, at least part of the
atom was to be composed of Thomson's particulate negative
corpuscles, although the rest of the positively charged part of the atom
remained somewhat nebulous and ill-defined.
The 1904 Thomson model was disproved by the 1909 gold foil
experiment of Hans Geiger and Ernest Marsden. This was interpreted by Ernest
Rutherford in 1911 to imply a very small nucleus of the atom containing a very
high positive charge (in the case of gold, enough to balance about 100
electrons), thus leading to the Rutherford model of the atom. Although gold
has an atomic number of 79, immediately after Rutherford's paper appeared in
1911 Antonius Van den Broek made the intuitive suggestion that atomic
number is nuclear charge. The matter required experiment to decide. Henry
Moseley's work showed experimentally in 1913 (see Moseley's law) that the
effective nuclear charge was very close to the atomic number (Moseley found
only one unit difference), and Moseley referenced only the papers of Van den
Broek and Rutherford. This work culminated in the solar-system-like (but
quantum-limited) Bohr model of the atom i the same year, in which a nucleus
containing an atomic number of positive charge is surrounded nby an equal
number of electrons in orbital shells. Bohr had also inspired Moseley's work.

10. •
•
•
Thomson's model was compared (though not by Thomson) to a British dessert called plum
pudding, hence the name. Thomson's paper was published in the March 1904 edition of
the Philosophical Magazine, the leading British science journal of the day. In Thomson's
view:
... the atoms of the elements consist of a number of negatively electrified corpuscles
enclosed in a sphere of uniform positive electrification, ...[4]
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 (see 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.

11. Ernest Ruthorford
• Ernest Rutherford, 1st Baron Rutherford of Nelson, OM FRS[1] (30
August 1871 – 19 October 1937) was a New Zealandborn physicist andchemist who became known as the father of nuclear
physics. He is considered the greatest experimentalist since Michael
Faraday (1791–1867).
• In early work he discovered the concept of radioactive half-life, proved
that radioactivity involved the transmutation of one chemical element
to another, and also differentiated and named alpha and beta
radiation.[3] This work was done at McGill University in Canada. It is
the basis for the Nobel Prize in Chemistry he was awarded in 1908 "for
his investigations into the disintegration of the elements, and the
chemistry of radioactive substances".[4]

12. •
•
Rutherford moved in 1907 to the Victoria University of
Manchester (today University of Manchester) in the UK, where he and Thomas
Roydsproved that alpha radiation was helium ions.] Rutherford performed his
most famous work after he became a Nobel laureate. In 1911, although he could
not prove that it was positive or negative, he theorized that atoms have their
charge concentrated in a very small nucleus, and thereby pioneered
the Rutherford model of the atom, through his discovery and interpretation
of Rutherford scattering in hisgold foil experiment. He is widely credited with
first "splitting the atom" in 1917 in a nuclear reaction between nitrogen and
alpha particles, in which he also discovered (and named) the proton.[9]
Rutherford became Director of the Cavendish Laboratory at Cambridge
University in 1919. Under his leadership the neutron was discovered by James
Chadwick in 1932 and in the same year the first experiment to split the nucleus
in a fully controlled manner, performed by students working under his
direction, John Cockcroft and Ernest Walton. After his death in 1937, he was
honoured by being interred with the greatest scientists of thee United
Kingdom, near Sir Isaac Newton's tomb in Westminster Abbey. The chemical
lement rutherfordium(element 104) was named after him in 1997

13. Ruthorford Modal of an atom
•
•
the prize. Along with Hans Geigerand Ernest Marsden in 1909, he carried out the Geiger–
MarsdeRutherford remains the only science Nobel Prize winner to have performed his most famous
work after receiving n experiment, which demonstrated the nuclear nature of atoms. Rutherford was
inspired to ask Geiger and Marsden in this experiment to look for alpha particles with very high
deflection angles, of a type not expected from any theory of matter at that time. Such
deflections, though rare, were found, and proved to be a smooth but high-order function of the
deflection angle. It was Rutherford's interpretation of this data that led him to formulate
the Rutherford model of the atom in 1911 – that a very small charged [7] nucleus, containing much of
the atom's mass, was orbited by low-mass electrons.
Before leaving Manchester in 1919 to take over the Cavendish laboratory in Cambridge, Rutherford
became, in 1919, the first person to deliberately transmute one element into another.[4] In this
experiment, he had discovered peculiar radiations when alphas were projected into air, and narrowed
the effect down to the nitrogen, not the oxygen in the air. Using pure nitrogen, Rutherford used
alpha radiation to convert nitrogen into oxygen through the nuclear reaction 14N + α →17O + proton.
The proton was not then known. In the products of this reaction Rutherford simply identified
hydrogen nuclei, by their similarity to the particle radiation from earlier experiments in which he had
bombarded hydrogen gas with alpha particles to knock hydrogen nuclei out of hydrogen atoms. This
result showed Rutherford that hydrogen nuclei were a part of nitrogen nuclei (and by
inference, probably other nuclei as well).

14. •
Such a construction had been suspected for many years on the basis of atomic
weights which were whole numbers of that of hydrogen; see Prout's hypothesis.
Hydrogen was known to be the lightest element, and its nuclei presumably the
lightest nuclei. Now, because of all these considerations, Rutherford decided
that a hydrogen nucleus was possibly a fundamental building block of all
nuclei, and also possiblyIn 1921, while working with Niels Bohr (who postulated
that electrons moved in specific orbits), Rutherford theorized about the
existence of neutrons, (which he had christened in his 1920 Bakerian
Lecture), which could somehow compensate for the repelling effect of the
positive charges of protons by causing an attractive nuclear force and thus
keep the nuclei from flying apart from the repulsion between protons. The only
alternative to neutrons was the existence of "nuclear electrons" which would
counteract some of the proton charges in the nucleus, since by then it was
known that nuclei had about twice the mass that could be accounted for if
they were simply assembled from hydrogen nuclei (protons). But how these
nuclear electrons could be trapped in the nucleus, was a mystery.

15. • Rutherford's theory of neutrons was proved in 1932 by his
associate James Chadwick, who recognized neutrons immediately when
they were produced by other scientists and later himself, in bombarding
beryllium with alpha particles. In 1935, Chadwick was awarded the
Nobel Prize in Physics for this discovery.
• a new fundamental particle as well, since nothing was known from the
nucleus that was lighter. Thus, Rutherford postulated hydrogen nuclei
to be a new particle in 1920, which he dubbed the proton.

16. Neils bohr
• Niels Henrik David Bohr ( 7 October 1885 – 18 November 1962) was a
Danish physicist who made foundational contributions to
understanding atomic structure and quantum theory, for which he
received the Nobel Prize in Physics in 1922. Bohr was also a
philosopher and a promoter of scientific research.
• Bohr developed the Bohr model of the atom, in which he proposed that
energy levels of electrons are discrete, and that they revolve in stable
orbits around the atomic nucleus, but can jump from one energy level
(or orbit) to another. Although the Bohr model has been supplanted by
other models, its underlying principles remain valid. He conceived the
principle of compiementarity: that items could be separately analysed in
terms of contradictory properties, like behaving as a wave or a stream
of particles. The notion of complementarity dominated his thinking on
both science and philosophy.

17. • Bohr founded the Institute of Theoretical Physics at the University of
Copenhagen, now known as the Niels Bohr Institute, which opened in
1920. Bohr mentored and collaborated with physicists including Hans
Kramers, Oskar Klein, George de Hevesy and Werner Heisenberg. He
predicted the existence of a new zirconium-like element, which was
named hafnium, after the Latin name for Copenhagen, where it was
discovered. Later, the element bohrium was named after him.
• During the 1930s, Bohr helped refugees from Nazism. After Denmark
was occupied by the Germans, he had a meeting with Heisenberg, who
had become the head of the German nuclear energy project. In
September 1943, word reached Bohr that he was about to be arrested
by the Germans, and he fled to Sweden. From there, he was flown to
Britain, where he joined the British Tube Alloys nuclear weapons
project, and was part of the British mission to the Manhattan Project.
After the war, Bohr called for international cooperation on nuclear
energy. He was involved with the establishment of CERN and
the Research Establishment Risø of the Danish Atomic Energy
Commission, and became the first chairman of the Nordic Institute for
Theoretical Physics in 1957

18. Bohr modal of an atom
•
•
The Rutherford–Bohr model of the hydrogen atom (Z = 1) or a hydrogen-like ion (Z > 1), where
the negatively charged electron confined to an atomic shell encircles a small, positively
charged atomic nucleusand where an electron jump between orbits is accompanied by an
emitted or absorbed amount of electromagnetic energy . The orbits in which the electron may
travel are shown as grey circles; their radius increases as n2, where n is the principal quantum
number. The 3 → 2 transition depicted here produces the first line of the Balmer series, and for
hydrogen (Z = 1) it results in a photon of wavelength 656 nm (red light). In atomic
physics, the Bohr model, introduced by Niels Bohr in 1913, depicts the atom as small, positively
charged nucleussurrounded by electrons that travel in circular orbits around the nucleus—
similar in structure to the solar system, but with attraction provided by electrostatic
forces rather than gravity. After the cubic model (1902), the plum-pudding
model (1904), the Saturnian model(1904), and the Rutherford model (1911) came the Rutherford–
Bohr model or just Bohr model for short (1913). The improvement to the Rutherford model is
mostly a quantum physical interpretation of it. The Bohr model has been superseded, but the
quantum theory remains sound.

19. •
•
The model's key success lay in explaining the Rydberg formula for the
spectral emission lines of atomic hydrogen. While the Rydberg formula had been
known experimentally, it did not gain a theoretical underpinning until the Bohr
model was introduced. Not only did the Bohr model explain the reason for the
structure of the Rydberg formula, it also provided a justification for its
empirical results in terms of fundamental physical constants.
The Bohr model is a relatively primitive model of the hydrogen atom, compared
to the valence shell 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.

20. Bohr modal