atoms truc lect 1

7/30/2019 Atoms Truc Lect 1

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Structure of the Atom


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Atoms ?~400 BC:

Greek Philosopher Democritus believed that each kind of

matter could be subdivided into smaller and smaller bits untilone reached the very limit beyond which no further divisionwas possible.

atomos = that cannot be cut

As of 1900, about 70 different atoms are knownelements 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 dont bind as molecules ?- New phenomena: X-rays, Radioactivity (1896),

Henri Becquerel 1852-1908, Nobel Prize in Physics 1903


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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.


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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 Becquerels 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

If X-rays were emitted, they would pass through the paper and fog the plate.

Tests are all negative, except for a Uranium Salt.

Sunlight

Substance

Photographic Plate

Black paper


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Becquerel ExperimentReproducibility: 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 thatthe 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.


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Search for elementsof 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 1903Also: Nobel Prize in Chemistry (1911)

Radium is so active that it shines brightly in its pure form

Marie Curie about Radium:its active and it radiates RADIOACTIVITY

She died of Leukemia at 67 (Exposure to radiations)


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The atomic models ofThomson and Rutherford

J.J.Thomson

1856-1940

Nobel Prize in Physics 1906

Ernerst Rutherford

1871-1937

Nobel Prize in Physics 1908


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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


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Thomsons atomic model Thomsons 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.

In Thomsons view, when the atom is heated, the electrons couldvibrate about their equilibrium positions, thus producingelectromagnetic radiation.

The model fails: cannot reproduce the spectral lines of the Hydrogen

atom

Positively charged, so thatthe whole atom is neutral


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Radioactivity ?

1898: Pierre and Marie discover theRadium Emission of -particle

1900: Rutherford and Roydsdetermine the nature of the -particles, they are charged Heliumatoms


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Ernest Rutherford 1871-1937Nobel Prize in Physics 1908


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Geiger and Marsden Experiments

1909: Rutherford, Geiger andMarsden conceive a new technique toprobe the structure of matter by

scattering -particles from atoms

Geiger shows that many -particlesare scattered from thin gold-leaftargets at backward angles greater

than 90


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Analysis / Conclusions

In contradiction with J.J.Thomsons model:

At best, -particles should only be slightlydeflected

Large deflections ? (see example 4.1, p129-130)

99.95% of the MASS of the atom lies in ahard, dense nucleus occupying only ~10-15mof 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 asmall volume, and that a large single deflections are due to the central

charge as a whole, and not to its constituents.


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Rutherford Scattering

Impact parameter b:

Note: the nucleus is 105 times smaller than the atom, large

deflections do not occur often !!!


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Failure of the classical(planetary) atomic model

Assuming the Hydrogen atom:

The electron is attracted by thenucleus

Even in circular motion around thenucleus, the electron loses energy:

Radial acceleration: ar = v2/R

Classical e.m. theory: an accelerating

charge continuously radiates energy, rdecreases

Atom (neutral) = nucleus (+q) + q electrons

The electron would eventually crash into the nucleus

atoms truc lect 1

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