A look at how the atom gained credibility in ancient times and how atomic theory was structured through experimentation.

1. The Evolution of the Atom: From a
Greek Belief to Its Modern Scientific
State Through Experimentation
?

2. A Greek Belief
Greeks where ineterested in knowing what the world
is made of.
• Thales ofofMiletus (ca.640-546 B.C.) What is
the nature matter?
• Leucippus (490-? B.C.) Can one endlessly devide
matter?

3. • Democritus (460-370the division of amatterof
Leuccipus. Called the limit of
B.C.) was pupil
atomos.
= the atom
Matter
He said everything, except the void, consists of a collection
of atoms.
~ Plato and Aristotle did not accept atoms.

4. • Epicurusteachings. B.C.) took atomism as central
core of his
(341-270
• Titus Lucretius Carus Natura. B.C.) was an
Epicurean. Wrote De Rerum
(96-55
Christianity accused atomists of atheism
• Pierreand adopted atomism. He shared the philosophy.
Natura
Gassendi (1592-1655) read De Rerum

5. The Collection of Evidence
Experimentation
• Robert Boyle (1627-1691) became an atomist
inlfuenced by Gassendi’s writings.
In 1662 Boyle proved the inverse relationship between pressure
and volume, which atomism explained easily.

6. • Joshep LouisLaw of definite proportions.in 1799
determined the
Proust (1754-1826)
Proust said that the atom
O is indivisible; it exist as
Cu O a whole piece or as
C nothing. This is also what
Democritus said.
O

7. • John Dalton determining the Law1803 built on
Proust’s Law by
(1766-1844) in
of multiple
proportions.With it he brought more credibility to atoms
which he named as a tribute to Democritus.
Carbon Monoxide arbon Dioxide
C
In 1808 Dalton published New System of Chemical
Philosophy with which he established the modern atomic
theory.

8. • RobertofBrownparticles he saw ininhis experimentthe
behavior pollen
(1733-1859) 1827 called
Brownian motion.
James Clerk Maxwell worked out the theory of constant motion of atoms
mathematically around 1860.
Albert Einstein worked out equations that described Brownian motion and
helped Jean Baptiste Perrin calculate the size of atoms.
• Erwin Wilhelmthat could magnify the pointinof1936
imagined a device
Mueller (1911-1977)
a fine
needle until seeing the atoms lined up as luminous dots.
In 1955 such atoms were seen.

9. • William Crookes (1832-1919) in 1878 built a
Crookes tube: an improved Geisser tube.
Made a little wheel inside
the tube turn when the
greenish glow hit it.
Was the greenish glow
particles or waves?

10. • Joseph John Thompson with a better vacuum tube
repeated Hertz’s experiment
(1856-1940) in 1897
and stronger charge plates. He convinced others that
cathode rays consisted of speeding cathode-ray
particles, each carrying a negative electric charge.
Cathode-ray particles are to electricity what an atom is to matter.
Thompson measured its mass to be significantly smaller than the
mass of the atom.

11. George Johnstone Stoney called the cathode-ray particles electrons.
Thompson referred to them as “corpuscles.”
In 1898 Thompson was the first to suggest an atomic structure taking
an electric charge into account.
Plum pudding model

12. • Hantarowith some things about Thompson’s model.
disagreed
Nagaoka (1865-1950) in 1904
Nagaoka believed the positive charge in the atom did
not take up as much volume as the electron and that it
was located in the atomic center.
• Ernest Rutherford (1871-1937) in 1909 directed
the Geiger-Mardsen experiment, better known as the
gold foil experiment, disproving Thompson’s atomic
model and proving the existence of an atomic nuleus.

13. Nucleus takes 99.945% to 99.975% of the atom’s mass

14. • Niels Bohr Rutherford’s planetary atomic but further
was based on
in 1903 proposed his
model
model which
explained.
Postulate I: electrons
revolved in circular orbits
where their angular
momentum is an integral
multiple of h/2 π. The
orbits are around the
nucleus and are stationary
states.
Postulate II: The energy of the atom has a definite value in a stationary orbit. If
an electron jumps from a high energy orbit, it emits a photon. In order for an
electron to jump from a lower energy orbit to a high ebergy orbit, it must
absorb energy from a source.

15. • James Chadwick (1891-1974) in 1932 her
performed the experiment Irene Joliot-Curie and
husband had been studying and thus dicovered the
neutron.

16. Sources