Democritus first proposed in 400 BC that matter is made of indivisible particles called atoms. This idea was ignored for over 2000 years. In the early 1800s, Dalton proposed that atoms are small, hard spheres that make up elements. Thomson's 1897 discovery of the electron led to his "plum pudding" model of atoms with positive charge and embedded electrons. Rutherford's 1909 gold foil experiment showed that atoms have a small, dense nucleus surrounded by empty space. Bohr's 1913 model depicted electrons orbiting the nucleus in fixed energy levels like planets around the sun. Modern atomic theory describes electrons as existing in probabilistic "clouds" or orbitals around the nucleus based on quantum mechanics.

1. Atomic
models

2. Atomos: Not to Be Cut
The History of Atomic Theory

3. Atomic Models
 This model of the
atom may look
familiar to you. This is
the Bohr model. In
this model, the
nucleus is orbited by
electrons, which are
in different energy
levels.
 A model uses familiar ideas to
explain unfamiliar facts
observed in nature.
 A model can be changed as
new information is collected.

4.  The atomic
model has
changed
throughout the
centuries,
starting in 400
BC, when it
looked like a
billiard ball →

5. Who are these men?
In this lesson, we’ll learn
about the men whose quests
for knowledge about the
fundamental nature of the
universe helped define our
views.

6. Democritus
 This is the Greek
philosopher Democritus
who began the search for
a description of matter
more than 2400 years
ago.
 He asked: Could
matter be divided into
smaller and smaller
pieces forever, or was
there a limit to the
number of times a
piece of matter could
be divided?
400 BC

7. Atomos
 His theory: Matter could
not be divided into
smaller and smaller
pieces forever, eventually
the smallest possible
piece would be obtained.
 This piece would be
indivisible.
 He named the smallest
piece of matter “atomos,”
meaning “not to be cut.”

8. Atomos
 To Democritus, atoms
were small, hard
particles that were all
made of the same
material but were
different shapes and
sizes.
 Atoms were infinite in
number, always
moving and capable
of joining together.

9. This theory was ignored and
forgotten for more than 2000
years!

10. Why?
 The eminent
philosophers
of the time,
Aristotle and
Plato, had a
more
respected,
(and
ultimately
wrong) theory.
Aristotle and Plato favored the earth, fire, air
and water approach to the nature of matter.
Their ideas held sway because of their
eminence as philosophers. The atomos idea
was buried for approximately 2000 years.

12. JOHN DALTON
 Early 1800’s
 Thought atoms were smooth, hard balls
that could not be broken into smaller
pieces.
 All elements are made of atoms.
 All atoms of the same element are
exactly alike and have same mass.
 An atom of one element cannot be
changed into an atom of a different
element.
 Atoms cannot be created nor destroyed,
only rearranged.
 Compounds are made from atoms of

13. JOHN DALTON
Atomic Model
Object used to represent
theory

14. The “Billiard Ball” Model
 proposed by John Dalton in 1804
 this theory proposed that matter was composed of small,
spherical particles
 but evidence was later gathered that matter was composed of
even smaller bits

15. J.J. THOMSON
 1897
 An atom consists of negative charges scattered
throughout a ball of positive charges.

16. J.J. THOMSON
Atomic Model
Object used to represent
theory

17. Thomson: “Plum Pudding” or
“Chocolate Chip Cookie” Model
 using available data on the atom, J.J. Thomson
came up with the idea of having charges embedded
with Dalton’s Billiard Balls
 Also used cathode ray experiment to discover the
existance of the electron
positive
(evenly distributed)
“dough”
part
negative
“chocolate”
note: this model kept Dalton’s key ideas intact

18. Thomson Model
 The discovery of the electron by J. J. Thomson showed that
atoms did have some kind of internal structure.
 The Thomson model of the atom described the atom as a
"pudding" of positive charge, with negatively charged
electrons embedded

19. Discovery of the Electron
In 1897, J.J. Thomson used a cathode ray
tube to deduce the presence of a negatively
charged particle: the electron
Click on me:
http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf

20. Conclusions from the Study of the
Electron:
A. Cathode rays have identical properties
regardless of the element used to produce
them. All elements must contain identically
charged electrons.
B. Atoms are neutral, so there must be
positive particles in the atom to balance the
negative charge of the electrons
C. Electrons have so little mass that atoms
must contain other particles that account
for most of the mass

21. J.J. Thomson’s Plum Pudding Model
Positively
charged
“pudding”
Negatively
charged particles
later named
electrons

22. ERNEST RUTHERFORD
 1911
 Rutherford was a student of Thomson.
 Positive charge (protons) is located in the center of
the atom.
 Center is called the nucleus.
 Almost all of the atom’s mass is located in the
nucleus.
 Atom is mostly empty space with the electrons
moving around the nucleus.

23. ERNEST RUTHERFORD
Atomic Model
Object used to represent
theory

24. Nuclear Model
 Ernest Rutherford discovered a huge flaw in
the previous concept of the atom during his
now famous gold foil experiment

25. Gold Foil Experiment
• Particles shot through thin sheet of gold
• Most shots went straight through
• A small amount were deflected
• Hence… The atoms must be made of mostly empty space with a
small dense nucleus

26.  If previous models were correct alpha particles would
have passed straight through the the gold
Further explanation of Nuclear Model

27.  Rutherford found that most (99%) of the alpha particles that he shot at the gold went
straight through
 From these experiments Rutherford concluded that the atom had a dense positive core,
with the rest composed of mostly empty space with the occasional negatively charged
electron
Nuclear Model
+
-
-
-
-
-
note: this model completely changed the definition of atom

28. Rutherford’s Findings
#1 The nucleus is small
#2 The nucleus is dense
#3 The nucleus is positively
charged
* Most of the particles passed right through
* A few particles were deflected
* VERY FEW were greatly deflected
“Like howitzer shells bouncing
off of tissue paper!”
Conclusions:

29. NEILS BOHR
 1913
 Bohr was a student of Thomson & Rutherford
 Electrons could only have specific amounts of
energy, leading them to move in certain orbits.
 This model is also compared to planets orbiting in
the solar system.

30. NEILS BOHR
Atomic Model
Object used to represent
theory

31. Bohr Model
 Niels Bohr proposed that electrons revolve
around the central positive nucleus (like
planets in the solar system)
negative electrons
3 positive protons

32. Bohr Model
 Bohr also suggested that the electrons can only
revolve in certain orbits, or at certain energy levels
(ie, the energy levels are quantized)
no energy level in between steps

33. The Modern Atom
 The modern atom is further defined by the works of these
scientists:
 Max Plank
 de Broglie
 Albert Einstein
 James Chadwick
 Heisenberg
 Erwin Schrodinger
 Electrons do not orbit the
nucleus like planets, rather
they can be anywhere in a
cloudlike region around the
nucleus.
 Electrons of the same energy
are in the same energy level.

34. Max Plank
Germany, 1918
 Energy is gained or lost in discrete
“packets” called quanta
 Calculated the amount of energy
and determined that it is a constant
 Plank’s Constant
 hv
 Founded quantum mechanics
theory
 He was also an accomplished
musician!

35. •Father of Quantum Physics
•Electrons absorb and emit
energy in discrete “packets”
called quanta
Max Plank

36. de Broglie, 1924
 Electrons move like waves and so have
properties of waves.
Albert Einstein
 Einstein was simultaneously working on the
photoelectric effect, the theory of relativity
and the energy-mass relationship.

37. JAMES CHADWICK
 1932
 Found that the atom has a particle that was
electrically neutral (neutron).

38. MODERN SCIENTISTS / JAMES CHADWICK
Atomic Model
Object used to represent
theory

39. Heisenberg and Schrodinger
 Found that Electrons live in fuzzy
regions or “clouds” not distinct
orbits
 Improved on Bohr’s findings
 Electron location can not be
predicted
 Quantum Mechanical Model

40. Heisenberg, 1925
 Heisenberg proposed that it is not possible to
know the position and momentum of an
electron at the same time.
 Heisenberg Uncertainty Principle

41. Erwin Schrödinger
 Electrons exist in specific orbitals and are assigned separate
quantum numbers

42. Quantum Mechanical Model
 the current understanding of the atom is based on
Quantum Mechanics
 this model sees the electrons not as individual
particles, but as behaving like a cloud - the electron
can be “anywhere” in a certain energy level
 Remember back to CPE with electrons behaving
like bees in a beehive

43. Quantum Mechanical Model
electrons can be found
anywhere in these “shells”
note: the electrons
are still quantized
no electrons can
be found here

44. FYI: Chemistry
 most things we do can be explained using
Dalton’s and/or Bohr’s model
 the Quantum Mechanical model, although
most accurate, is complex even at a
university level (conceptually and
mathematically)
 Click on me or copy and paste into browser:
http://www.hyperhistory.com/online_n2/people_n2/science_n2/atomic_theory.html

45. The Modern Atomic View
The Wave-Mechanical Model

46. Another View

47. Wave Model

48. The Wave Model
 Today’s atomic model is
based on the principles of
wave mechanics.
 According to the theory of
wave mechanics,
electrons do not move
about an atom in a definite
path, like the planets
around the sun.

49. The Wave Model
 In fact, it is impossible to
determine the exact location of an
electron. The probable location of
an electron is based on how much
energy the electron has.
 According to the modern atomic
model, at atom has a small
positively charged nucleus
surrounded by a large region in
which there are enough electrons
to make an atom neutral.

50. Electron Cloud:
 A space in which electrons are
likely to be found.
 Electrons whirl about the nucleus
billions of times in one second
 They are not moving around in
random patterns.
 Location of electrons depends upon
how much energy the electron has.

51. Electron Cloud:
 Depending on their energy they are locked into a certain area in
the cloud.
 Electrons with the lowest energy are found in the energy
level closest to the nucleus
 Electrons with the highest energy are found in the outermost
energy levels, farther from the nucleus.

52. Indivisible Electron Nucleus Orbit Electron
Cloud
Greek X
Dalton X
Thomson X
Rutherford X X
Bohr X X X
Wave X X X

53. Models of the Atom
Dalton’s model
(1803)
Thomson’s plum-pudding
model (1897)
Rutherford’s model
(1909)
Bohr’s model
(1913)
Charge-cloud model
(present)
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 125
Greek model
(400 B.C.)
+
-
-
-
-
-
e
e
e
+
+ +
+
+
+
+
+
e
e
e
e
e
e
e
"In science, a wrong theory can be valuable and better than no theory at all."
- Sir William L. Bragg

54. Models of the Atom
Dalton’s model
(1803)
Thomson’s plum-pudding
model (1897)
Rutherford’s model
(1909)
Bohr’s model
(1913)
Charge-cloud model
(present)
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 125
Greek model
(400 B.C.)
1800 1805 ..................... 1895 1900 1905 1910 1915 1920 1925 1930 1935 1940 1945
1803 John Dalton
pictures atoms as
tiny, indestructible
particles, with no
internal structure.
1897 J.J. Thomson, a British
scientist, discovers the electron,
leading to his "plum-pudding"
model. He pictures electrons
embedded in a sphere of
positive electric charge.
1904 Hantaro Nagaoka, a
Japanese physicist, suggests
that an atom has a central
nucleus. Electrons move in
orbits like the rings around Saturn.
1911 New Zealander
Ernest Rutherford states
that an atom has a dense,
positively charged nucleus.
Electrons move randomly in
the space around the nucleus.
1913 In Niels Bohr's
model, the electrons move
in spherical orbits at fixed
distances from the nucleus.
1924 Frenchman Louis
de Broglie proposes that
moving particles like electrons
have some properties of waves.
Within a few years evidence is
collected to support his idea.
1926 Erwin Schrödinger
develops mathematical
equations to describe the
motion of electrons in
atoms. His work leads to
the electron cloud model.
1932 James
Chadwick, a British
physicist, confirms the
existence of neutrons,
which have no charge.
Atomic nuclei contain
neutrons and positively
charged protons.
+
-
-
-
-
-
e
e
e
+
+ +
+
+
+
+
+
e
e
e
e
e
e
e