Download Atomic Structure Development

A History of Atomic Structure
Development
(To 1939)
First Atomic Concept
Democritus
Boyle (1627-1691) First modern chemist; introduced ‘atomism’;
every phenomenon is the results of collisions of particles in motion
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The Atom Defined
John Dalton (1766 – 1844) father of the atomic concept.
The main points of Dalton's atomic theory were:
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Elements are made of extremely small particles called atoms.
Atoms of a given element are identical in size, mass, and other
properties; atoms of diferent elements difer in size, mass, and other
properties.
Atoms cannot be subdivided, created, or destroyed.
Atoms of diferent elements combine in simple whole-number ratios
to form chemical compounds.
In chemical reactions, atoms are combined, separated, or rearranged
Defined atomic weight (from combining power with the lightest known
element – hydrogen - assumed to have atomic weight of 1)
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A New System of Chemical Philosophy
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Atoms – Real or Imaginery?
Helmholtz 1821 – 1894 Clausius 1822 - 1888 Boltzmann 1844 – 1902
Ostwald 1853 - 1932
Kekule 1829 – 1896
Kelvin 1824 - 1907
Clerk Maxwell 1831- 1879
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Periodic Table of the Elements
Mendeleev. (1834 – 1907)
Married at age 28, divorced 1882, married again 1 month before
divorce finalised
In 1863 – 65 arranged all the known elements in order of increasing
atomic weights setting AW of hydrogen as 1.
predicted the existence of undiscovered elements to fll gaps in the
table;
discoveries of gallium (1876), scandium (1879) and germanium
(1886) led to wide acceptance of the table.
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Cathode Rays
Sir William Crookes (1832 – 1919)
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J J Thomson and the electron
J J Thomson (1856 – 1940)
Intnl Physics Union asked JJ to determine nature of
cathode rays;
deflected by both E and M felds;
Rays attracted to +ve plate;
M field at right angles brought beam back to zero
deflection.
Calculates ratio of charge to mass.
Modern value 1.7588×1011 C/kg
1906 Nobel Prize in Physics for this work
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Millikan and electron charge
R A Millikan (1868 – 1953)
Nobel Prize in Physics 1923
Electron charge 1.592×10-6C
“There is no likelihood man can ever tap the power of the atom. The glib supposition of
utilizing atomic energy when our coal has run out is a completely unscientifc Utopian
dream, a childish bug-a-boo. "
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Radioactivity
Pierre and Marie Curie (1867 – 1934)
Nobel Prize in Physics 1903
PhD project on ‘Becquerel rays from pitchblende’ –
Identified uranium as source of rays; introduced
Term ‘radioactivity’
Identified two further radioactive elements;
Polonium (July 1898)
Radium (December 1898)
Second Nobel prize (Chemistry) 1911
Identified the 3 components of radioactive ‘rays’;
α rays – +ve charge stopped by a piece of paper;
β rays – -ve charge stopped by metal foil;
γ rays – no charge , high energy required several
cm of lead to stop them.
Concluded - radioactive elements transmutated
from one element to another.
Introduced half-life, number of decaying atoms ∝
total no of radioactive atoms available.
Rutherford (1871 -1937) Soddy (1877 -1956)
N=N0exp(-λt); t1/2= 0.693/λ13
1907 – Rutherford moves to Manchester
1908 ‘the beautiful experiment’ with Royds
Thomas Royds (1884 – 1955)
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The Quantum Atom
Heating a solid causes it to emit radiation – Black Body Radiation
– wavelength varies in characteristic way with T
Using kinetic theory and equipartition of energy to
calculate spectrum of emitted radiation
Max Planck (1858 –
1947)
Nobel Prize in Physics
1918
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Atomic Structure Models
Elements when heated or subjected to
electric discharge emit radiation in
sharp frequencies - Line spectra
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Plum Pudding Model of Atom Structure
Proposed by J J Thomson in 1904– electrons moved in orbits ;
stabilised by interactions between them.
Diferent orbits had diferent energies;
heat and/or light caused atoms to collide.
Electrons vibrated about their orbits – emitting radiation
of same frequency as vibrations’
Model failed to predict observed line spectra.
hydrogen-atom.htm
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Geiger-Marsden Experiment (1909)
Hans Geiger(1882 – 1945)
Ernest Marsden (1889 -1970)
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‘it was as if you fired a 15 inch shell at tissue paper and it came back and hit you’
hydrogen-atom.htm
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Rutherford – Bohr Model of the Atom
PhD 1911 – joined Thomson’s group in Cavendish lab – not a happy
match – went to Rutherford in Manchester.
Bohr used the essential feature of the Rutherford model ;
confined attention to the simplest atom – hydrogen.
Niels Bohr (1885 – 1962)
Nobel Prize in Physics 1922
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Zeeman effect is the splitting of spectral lines
when a magnetic field is applied.
spectral lines are doublets, even in the
absence of an external magnetic field
Four quantum numbers define electron orbital
n – Principal – energy level
l – angular momentum
m – magnetic quantum number
s – spin
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Wave Mechanics and refnements to Bohr model
De Broglie (1892 – 1987)
Particle
Electron ac by
100V
Acd by 10000V
∝ particle from
radium
Golf ball
mass/kg
9.11×10 -31
speed /ms -1
5.9×106
λ /m
1.2×10 -10
9.29×10-31
6.68×10-27
5.9×107
1.5×107
0.12×10-10
6.6×10-15
0.045
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4.9×10-34
Rationalised Bohr’s quantised electron orbits – electron wave must be
in phase when electron completes a revolution of the orbit.
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hydrogen-atom.htm
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Nobel Prize in Physics for 1937 shared by
Thomson and Davisson
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Erwin Schrödinger
(1887 -1961)
Nobel Prize in Physics
1933
hydrogen-atom.htm
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Atomic Structure in the 1920s
Worked with Rutherford 1910 – 1913 – returned to Oxford
Bombarded various elements with electrons to produce X-rays.
Henry Moseley (1887 – 1915)
Dependence of the square root of the
frequency on atomic number Z could be
explained in terms of the Bohr theory
There were gaps in the plot, corresponding to
the atomic numbers 43, 61 and 75.
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1919 Rutherford moved to Cambridge as Cavendish professor –Rutherford brought Chadwick
with him from Manchester and recruited Blackett; Appleton; Cockcroft and Walton
amongst others.
Much of the efort in atomic structure was on theoretical description of electron
distribution; Bohr; de Broglie; Pauli; Heisenberg; Schrodinger
Thomson and Rutherford – atoms known to consist of electrons surrounding a
small nucleus containing protons
- number of protons = number of electrons = Atomic Number, Z, of the element.
large deficit in mass when compared with known atomic weights.
E.g. Gold Z=79; AW=197
One idea was that the additional mass was made up by ∝ particles
Rutherford had discussed possibility of a neutral particle made up of a proton and an
electron - attempts to prove this had led to no success.
– but the idea of a neutral particle was attractive.
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Chadwick and the Neutron
For some years Chadwick and Rutherford had
debated the existence of a neutral particle in
the atomic nucleus – but had yet to discover it.
James Chadwick
(1891 – 1974)
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Observations not reconcilable with production of γ rays
‘The difculties disappear however, if it be assumed that the
radiation consists of particles of mass 1 and charge 0, or neutrons’
Letter in Nature in Feb 1932
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Splitting the Atom
Bombarding matter with high energy particles
accepted means of smashing the atomic nucleus.
Natural sources of e.g ∝ particles – limited energy
Rutherford saw need to accelerate +ve particles
Through HV field – 8MV calculated.
Cockcroft had served in WW1 – not an able
experimentalist – but a good ‘fxer’
Walton was Irish – PhD project to build a circular
accelerator
of electrons 1927– but finding it difficult
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‘We might in these processes obtain very much more energy than the proton supplied,
but on the average we could not expect to obtain energy in this way. It was a very poor
and inefcient way of producing energy, and anyone who looked for a source of power
in the transformation of the atoms was talking moonshine’ Rutherford BA speech 1932
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Nuclear Fission
Otto Hahn(1879-1968)
Lise Meitner (1878 – 1968)
Otto Frisch (1904-1979)
Enrico Fermi (1901-1954)
‘But that’s another story’
Moustache in Irma la Douce.
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SOURCES
Physical Chemistry a Molecular Approach
D A McQuarrie and J D Simon
Walther Nernst and the transition to modern physical science
Niels Bohr and the quantum atom
The Fly in the Cathedral
The Elements
Science in the 20th Century
Nobel Prize Lecture
Diana Barkan
Helge Kragh
B Cathcart
Jack Challoner
John Krige, Dominique Pestre
F Perrin
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1934 began experimental work on neutron bombardment of matter;
light elements transmuted to lighter elements by ejecting either a proton or an alpha.
Heavy elements lost energy by emission of a gamma ray and formation of a heavier
isotope;
Uranium – emitted a beta ray (electron) – several diferent half-lives observed –
suggestion that new man made element Z= 93 formed.
Awarded 1938 Nobel Physics prize "for his demonstrations of the existence of new
radioactive elements produced by neutron irradiation, and for his related discovery of
nuclear reactions brought about by slow neutrons".
In 1938 Hahn and Meitner had been collaborating for 30 yrs - transmutation products
of
uranium.
Hahn repeated Curie expts - 3 new isotopes thought to be of radium.
Repeated again – the radium was in fact barium – but how?
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Otto Frisch visited Meitner– Christmas 1938;
‘liquid drop’ model of atomic nucleus due to Bohr and Gamow;
Exptl paper from Hahn appeared January 1939. 1944 Nobel Prize in Chemistry
Frisch – additional expts in Bohr institute confirmed energy release estimates
– theory paper to Nature Feb 1939;
Frisch tells Bohr in early January 1939– on his way to conference in USA.
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Shell model of the atom
Approximate model for heavier atoms.
Each discrete orbit can only hold a certain number of electrons.
After that orbit is full, the next level is used; giving the atom a shell structure. Each shell
corresponds to a Bohr orbit.
The shell model qualitatively explains properties of atoms codified in the periodic table.
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Atoms on the right of the table tend to gain electrons.
Atoms to the left tend to lose electrons
Every element on the last column of the table is chemically inert (noble gas); due to shellfilling.
The first Bohr orbit is filled when it has two electrons.
The second orbit can take eight electrons, it is full at neon.
The third orbit contains 18 electrons.
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K
Na
Li
Kr
Ar
Ne
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