This document provides an overview of the structure of an atom according to different atomic models over time. It begins with Dalton's atomic theory from 1807 that atoms are fundamental units of matter. Later models include Thomson's "plum pudding" model from 1897, Rutherford's discovery of the nucleus from experiments in 1908, and Bohr's model from 1913 which proposed electrons orbit in distinct energy levels. The document then discusses the subatomic particles of protons, neutrons, and electrons and their roles. It concludes with definitions of related atomic concepts such as atomic number, mass number, isotopes, and isobars.

2. AUM
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3. AUM
AMRITHESHWARYAI
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4. TO OUR PRESENTATION

5. STRUCTURE OF AN ATOM
CHEMISTRY GROUP WORK

7. TITLE Slide
• INTRODUCTION 8-10
• CONTENT 11-50
-What Is An Atom? 12-14
-Thomsons Model 15-19
-Discovery Of Protons 20-21
-Discovery Of Neutrons 22-23
-Rutherfords Model 24-33
-Bohrs Model 34-40
-Subparticles In Atom 41-46
-Valency 47
- Atomic Number 48
-Mass Number 49
-Isotopes 50-51
-Isobars 52
• CONCLUTION 53-54
• TEAM MEMBERS 55
• REFERENCE 56

8. INTRODUCTION
Atom was discovered by John Dalton. He
proposed the famous atomic theory in
1807. Atoms are fundamental unit of
matter. The existence of different kinds of
matter is due to different atoms constituting
them.

9. A major challenge before the scientists at the
end of the 19th century was to reveal the
structure of the atom as well as to explain its
important properties. Many scientists worked
hard and proposed many models for the
atom, here we are going to learn about the
structure of an atom

10. STRUCTURE OF AN ATOM
The discovery of the two fundamental particles
(electrons and protons) inside the atoms led
to the failure of the aspect of Daltons atomic
theory. For explaining the arrangement of
electrons and protons in an atom, many
scientists proposed various atomic models.

11. CONTENT

14. All matter is made up of atoms.
Atoms of an element are
identical.
Each element has different atoms.
Atoms can engage in a chemical
reactions.
Atoms can neither be created nor
be destroyed.
Atoms are indivisible.
Atomic Theory
John Dalton
(1776-1884)

15. Thomson’s Plum Pudding Model
In 1897, the English
scientist J.J.Thomson
provided the first hint
that an atom is made
of even smaller
particles.

16. Thomson Model
• He proposed a model of the atom
that is sometimes called the “Plum
Pudding” model.
• Atoms were made from a positively
charged sphere with negatively
charged electrons embedded in it,
like raisins in a pudding.
• The negative and positive charges
are equal in magnitude.So,the atom
as a whole is electrically stable

17.  Passing an electric current makes a beam
appear to move from the negative to the
positive end.
Thomson’s Experiment And
Discovery of Electrons
Voltage source +-

18. Voltage source
Thomson’s Experiment
 By adding an electric field he found that
the moving pieces were negative. He
called these moving pieces “electron”
+
-

19. The electron was discovered in 1897 by
Thomson. He imagined the atom as a
“raisin pudding” with electrons stuck in a
cake of positive charge.

20. Discovery of Protons
• Eugene Goldstein noted streams of
positively charged particles in cathode
rays in 1886.
–Particles move in opposite direction
of cathode rays.
–Called “Canal Rays” because they
passed through holes (channels or
canals) drilled through the negative
electrode.

21. Canal rays must be positive.
Goldstein postulated the
existence of a positive
fundamental particle called
the “proton”.

22. DISCOVERY OF NEUTRONS
In 1932, J. Chadwick discovered another subatomic
particle which had no charge and a
mass nearly equal to that of a proton. It was
eventually named as neutron. Neutrons are
present in the nucleus of all atoms, except
hydrogen. In general, a neutron is
represented as ‘n’. The mass of an atom is
therefore given by the sum of the masses of
protons and neutrons present in the nucleus.

23. NEUTRONS

24. Rutherford’s Gold Foil Experiment
• In 1908, the English
physicist Ernest
Rutherford was hard
at work on an
experiment that
seemed to have little
to do with
unraveling the
mysteries of the
atomic structure.

25. • Rutherford’s experiment Involved firing a
stream of tiny positively charged alpha (α)
particles at a thin sheet of gold foil.
• The expected result was that the α-
particles would be deflected by the sub-
atomic particles in gold atoms. Since α -
particles were much heavier than protons,
he did not expect to see larger deflections

29.  Most of the positively charged “bullets” passed right
through the gold atoms in the sheet of gold foil
without changing course at all.
 Some of the positively charged “bullets,” however, did
bounce away from the gold sheet as if they had hit
something solid. He knew that positive charges repel
positive charges.

30.  This could only mean that the gold atoms in the
sheet were mostly open space. Atoms were not a
pudding filled with a positively charged material.
 Rutherford concluded that an atom had a small,
dense, positively charged center that repelled his
positively charged “bullets.”
 He called the center of the atom the “nucleus”
 The nucleus is tiny compared to the atom as a
whole.

31. Rutherford’s Postulates
Rutherford reasoned that
all of an atom’s
positively charged
particles were
contained in the
nucleus. The negatively
charged particles were
scattered outside the
nucleus around the
atom’s edge.

32. Solar system Rutherford‘s atom model
Planetary model of atom

33. The model created by Rutherford had still some
serious discordance. According to the classic
science, electron moving around the nucleus or
the planetary model of atom,is unstable.
Because ,in the planetary model of atom, the
electron should radiate energy and finally fall in
to the nucleus.

34. Bohr’s Model
In 1913, the Danish
scientist Niels
Bohr proposed an
improvement. In
his model, he
placed each
electron in a
specific energy
level.

35. Following Rutherford’s planetary model of
the atom, it was realized that the attraction
between the electrons and the protons
should make the atom unstable
Bohr proposed a model in which the
electrons would stably occupy fixed orbits,
as long as these orbits had special discrete
locations

36. Bohr’s Model
Nucleus
Electron
Orbit
Energy Levels

37. Bohr's Model of the Atom
 The special orbits known as discrete orbits of
electron are allowed inside an atom. These
orbits or shells are represented by the letters
K,L,M,N,….. Or the numbers ,n=1,2,3,4,….

38. Bohr's Model of the Atom
 electrons fills the orbits closest to the
nucleus
e.g. fluorine:
#P = 9
#e- = 9
#N = 10
9P
10N
F
19.00
9

39. Each orbit can hold a specific maximum
number of electrons An easy way to calculate
the total number of electrons that can be held
by a given energy level is to use the formula
2*n2
Shell maximum no: of
electrons
1 - K 2*12=
2 - L 2*22=
3 - M 2*32=
4 - N 2*42=
Bohr's Model of the Atom
2
8
18
32

40. Key aspects of Bohr’s model
• Electrons move around the nucleus at
stable orbits without emitting radiation.
• Electron in one of these stable orbit has a
definite energy.
• Energy is radiated only when electrons
make transitions from high energy orbit to
a low energy orbit.

42. What is in the structure of an atom?
• Nucleus - center of the atom
Home of Protons and Neutrons
Has a positive charge
• Proton
Has a relative mass of 1u
Has a positive (+) charge
Determines the atomic number
Found inside the nucleus

43. What is in the structure of an atom?
• Neutron
Has no (0) charge
Has a relative mass of 1u
Found inside the nucleus

44. What is in the structure of an atom?
• Electron
Has a negative (-) charge
Found outside the nucleus
• Rutherford atom model - electrons are
around the nucleus
• Bohr model – electrons are in specific
energy levels called shells

45. How are p, n, e related?
• No. protons = No. electrons
• No. protons = atomic number
• No. protons + No.neutrons = mass number
• No. neutrons=mass no. - atomic no.

47. • The outermost shell of an atom is known as its
valence shell.
• The electrons present in the outermost shell
of an atom are known as the valence electrons
• The Valency of an element may be defined as
the combining capacity of its atoms with
atoms of other elements in order to acquire
octet configuration.
• For eg; The valency of hydrogen is 1.

48. • The number of protons in an atom is referred
to as its Atomic Number.
• It is denoted by the letter ‘ Z ’.
• Elements are defined by the number of
protons they posses.
• The atomic number of hydrogen is 1.

49. • The mass of an atom resides in its nucleus.
The mass of an atom is practically due to
protons and neutrons alone.
• Therefore, Mass Number of an atom is the
sum of neutrons and protons present in the
nucleus of an atom
• for hydrogen, its mass number is 1u.

50. • Isotopes are atoms of same element, which
have different mass numbers but same atomic
number.
• Their chemical properties are similar but
physical properties are different.

51. • An isotope of uranium is used as a fuel in
nuclear reactors.
• An isotopes of cobalt is used in the treatment
of cancer.
• An isotope of iodine is used in the treatment
of goitre.

52. • Atoms of different elements with different
atomic numbers ,which have the same mass
number are known as isobars.
• Examples of isobars are: calcium(z=20)and
argon(z=18).their mass number is 40 u.

53. CONCLUSION
• Atoms are the building blocks of matter.
• Thomsons atom model is that atom is a
positively charged sphere and the electrons
are embedded in it and atoms are neutral.
• Rutherford did the famous gold foil
experiment and said that there is a positively
charged centre in an atom called nucleus and
the electrons revolve around the nucleus in
circular paths.

54. • Bohr was the next person. He proposed that only
special discrete orbits are allowed inside an atom and
while revolving electrons doesn’t radiate energy.
• The sub particles in a nucleus is electron, proton and
neutron.
• Valency is the combining capacity of atoms.
• Atomic number is the number of proton
• Mass number is the sum of the number of neutron and
protons in an atom.
• Isotopes are atoms of the same element, which have
different mass number and same atomic number.
• Isobars are atoms which has same mass number but
different atomic number,

55. TEAM MEMBERS

56. REFERENCE
• Class IX Science Text
• Wikipedia
• Google

60. JOHN DALTON
John Dalton was born in
a poor weaver’s family in
1766 in England. He
began his career as a
teacher at the age of
twelve. Seven years later
he became a school
principal. In 1793, Dalton
left for Manchester to
teach mathematics,
physics and chemistry in
a college. He spent most of his life there
teaching and researching. In 1808, he
presented his atomic theory which was a
turning point in the study of matter.
Back

61. J.J.Thomson
J.J. Thomson (1856-
1940), a British
physicist, was born in
Cheetham Hill, a suburb
of Manchester, on
18 December 1856. He
was awarded the Nobel
prize in Physics in 1906
for his work on the
discovery of electrons.
He directed the Cavendish Laboratory at
Cambridge for 35 years and seven of his
research assistants subsequently won
Nobel prizes.
Back

62. E .Rutherford
Ernest Rutherford,
(30 August 1871 – 19 October 1937) was a New Zealand-born British
physicist who became known as the father of “nuclear physics .He was
born at Spring Grove
on 30 August 1871. He was
known as the ‘Father’ of
nuclear physics. He is
famous for his work on
radioactivity and the
discovery of the nucleus of an atom with
the gold foil experiment. He got the
Nobel prize in chemistry in 1908.
Back

63. NEILS BOHR
• Niels Henrik David Bohr (7 October 1885 – 18 November
1962) was a Copenhagen born 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.
• He was appointed professor of physics at Copenhagen
University in 1916. Among Professor Bohr’s numerous
writings, three appearingas books are:
(i) The Theory of Spectra and Atomic Constitution,
(ii) Atomic Theory and,
(iii) The Description of Nature. Back