A History of Atomic Structure Development

(To 1939)

First Atomic Concept

Boyle (1627-1691) First modern chemist; introduced ‘atomism’; every phenomenon is the results of collisions of particles in motion

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Democritus

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John Dalton (1766 – 1844) father of the atomic concept.

The main points of Dalton's atomic theory were:

• 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)

The Atom Defined

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A New System of Chemical Philosophy

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Atoms – Real or Imaginery?

Clausius 1822 - 1888 Boltzmann 1844 – 1902

Kekule 1829 – 1896

Helmholtz 1821 – 1894

Ostwald 1853 - 1932 Clerk Maxwell 1831- 1879

Kelvin 1824 - 1907

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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.

Periodic Table of the Elements

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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/kg1906 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

“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. "

Electron charge 1.592×10-6C

RadioactivityPierre and Marie Curie (1867 – 1934)Nobel Prize in Physics 1903 PhD project on ‘Becquerel rays from pitchblende’ –

Identified uranium as source of rays; introducedTerm ‘radioactivity’

Identified two further radioactive elements;Polonium (July 1898)Radium (December 1898)Second Nobel prize (Chemistry) 1911

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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.

N=N0exp(-λt); t1/2= 0.693/λRutherford (1871 -1937) Soddy (1877 -1956)

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1907 – Rutherford moves to Manchester1908 ‘the beautiful experiment’ with Royds

Thomas Royds (1884 – 1955)

The Quantum AtomHeating 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

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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

Niels Bohr (1885 – 1962)Nobel Prize in Physics 1922

PhD 1911 – joined Thomson’s group in Cavendish lab – not a happymatch – went to Rutherford in Manchester.

Bohr used the essential feature of the Rutherford model ; confined attention to the simplest atom – hydrogen.

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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 orbitaln – Principal – energy levell – angular momentumm – magnetic quantum numbers – spin

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Wave Mechanics and refnements to Bohr model

De Broglie (1892 – 1987)

Particle mass/kg speed /ms -1 λ /mElectron ac by 100V

9.11×10 -31 5.9×106 1.2×10 -10

Acd by 10000V 9.29×10-31 5.9×107 0.12×10-10∝ particle from radium

6.68×10-27 1.5×107 6.6×10-15

Golf ball 0.045 30 4.9×10-34

Rationalised Bohr’s quantised electron orbits – electron wave must be in phase when electron completes a revolution of the orbit.

23hydrogen-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 Physics1933

hydrogen-atom.htm

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Henry Moseley (1887 – 1915)

Worked with Rutherford 1910 – 1913 – returned to OxfordBombarded various elements with electrons to produce X-rays.

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.

Atomic Structure in the 1920s

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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 asmall 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 anelectron - 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

James Chadwick(1891 – 1974)

For some years Chadwick and Rutherford haddebated the existence of a neutral particle inthe atomic nucleus – but had yet to discover it.

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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 acceleratorof 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 Diana Barkan

Niels Bohr and the quantum atom Helge Kragh

The Fly in the Cathedral B Cathcart

The Elements Jack Challoner

Science in the 20th Century John Krige, Dominique Pestre

Nobel Prize Lecture F Perrin

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".

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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 ChemistryFrisch – 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 atomApproximate 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.

• 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 shell-

filling. • 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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Ne

Ar

Kr

Li

Na

K