Structure of the Atom
Atoms ?
~400 BC:
Greek Philosopher Democritus believed that each kind of
matter could be subdivided into smaller and smaller bits until
one reached the very limit beyond which no further division
was possible.
“atomos” = “that cannot be cut”
As of 1900, about 70 different “atoms” are known
elements different chemical properties
it is argued that this number is too large to really be an elementary
constituant of matter
Hints:
- Atoms and electromagnetic radiations (not understood but…)
- Chemistry: the problem of valence. How molecules are formed ?
Why some combinations of atoms just don’t bind as molecules ?
- New phenomena: X-rays, Radioactivity (1896),…
Henri Becquerel 1852-1908, Nobel Prize in Physics 1903
Inside the atom ?
• Scientists are struggling to understand the atom and
reproduce experimental results, such as the spectral lines.
[Physics of the electrons inside the atom]
Atomic Physics
• Also puzzled with a new observed phenomenon:
radioactivity. [Physics of the nucleus]
Nuclear Physics
• But, around 1900, nobody knows the structure of the atom:
a puzzle of evidences that needs to be put together.
Discovery of Radioactivity (1896)
1895:
X-rays are interpreted as a chemical process similar to phosphorescence
and fluorescence
An external source is required to “trigger” the emission of X-rays
Henri Becquerel’s idea:
look for X-ray emission in known phosphorescent/fluorescent substances.
Experimental procedure:
wrap a photographic plate with thick black paper, place the substance to be
tested on the paper and then expose to sunlight for several hours
Sunlight = external source
Sunlight
Substance
Photographic Plate
Black paper
If X-rays were emitted, they would pass through the paper and fog the plate.
Tests are all negative, except for a Uranium Salt.
Becquerel Experiment
Reproducibility: an experimental result should be reproducible
End of February, Henri Becquerel is ready to repeat the experiment.
But it is cloudy over Paris !!!
Put his experimental setup in a drawer till March 1st.
When Becquerel develops the plate (not exposed to sunlight), he finds that
the fogging is just as intense as when the uranium salt had been exposed to sunlight.
No need for external energy source !!!
The energy is already available/stored in the material.
Search for elements
of similar properties
Pierre 1859-1906 and Marie Curie: Isolation of Radium and Polonium (1898)
Nobel Prize in Physics in 1903
Marie Curie 1867-1934:
Got her PhD in 1903
Also: Nobel Prize in Chemistry (1911)
Radium is so active that it shines brightly in its pure form
Marie Curie about Radium:
“it’s active and it radiates” RADIOACTIVITY
She died of Leukemia at 67 (Exposure to radiations)
The atomic models of
Thomson and Rutherford
J.J.Thomson
1856-1940
Ernerst Rutherford
1871-1937
Nobel Prize in Physics 1906
Nobel Prize in Physics 1908
Known facts (~1900)
• Atoms are neutral
• Electrons are much less massive than the atom
• Number of electrons Ne- corresponds to about half the
atomic mass number. Example:
– Carbon: Atomic Mass Number = 12; Ne-=6
– Oxygen: Atomic Mass Number = 16; Ne-=8
• Size of the atom ~ 10-10m
• The atoms can emit and absorb electromagnetic radiations
Thomson’s atomic model
• Thomson’s “plum-pudding” model of the atom had the positive
charges spread uniformly throughout a sphere the size of the atom,
with electrons embedded in the uniform background.
Positively charged, so that
the whole atom is neutral
• In Thomson’s view, when the atom is heated, the electrons could
vibrate about their equilibrium positions, thus producing
electromagnetic radiation.
• The model fails: cannot reproduce the spectral lines of the Hydrogen
atom
Radioactivity ?
• 1898: Pierre and Marie discover the
Radium Emission of α-particle
• 1900: Rutherford and Royds
determine the nature of the αparticles, they are charged Helium
atoms
Ernest Rutherford 1871-1937
Nobel Prize in Physics 1908
Geiger and Marsden Experiments
• 1909: Rutherford, Geiger and
Marsden conceive a new technique to
probe the structure of matter by
scattering α-particles from atoms
• Geiger shows that many α-particles
are scattered from thin gold-leaf
targets at backward angles greater
than 90º
Analysis / Conclusions
• In contradiction with J.J.Thomson’s model:
– At best, α-particles should only be slightly
deflected
• Large deflections ? (see example 4.1, p129-130)
– 99.95% of the MASS of the atom lies in a
hard, dense nucleus occupying only ~10-15m
of the atomic volume.
Rutherford (1911): ”Considering the evidence as a whole, it seems simplest
to suppose that the atom contains a central charge distributed through a
small volume, and that a large single deflections are due to the central
charge as a whole, and not to its constituents”.
Rutherford Scattering
Impact parameter b:
Note:
the nucleus is 105 times smaller than the atom, large
deflections do not occur often !!!
Failure of the classical
(planetary) atomic model
•Atom (neutral) = nucleus (+q) + q electrons
• Assuming the Hydrogen atom:
– The electron is attracted by the
nucleus
– Even in circular motion around the
nucleus, the electron loses energy:
• Radial acceleration: ar = v2/R
• Classical e.m. theory: an accelerating
charge continuously radiates energy, r
decreases…
The electron would eventually crash into the nucleus