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Atomic Structure
A level atThe Sixth Form College
Colchester
Adapted from:
An example of How Science Works:
Development of the Model of
Atomic Structure
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Early Ideas of Matter
Polystyrene foam?
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Early Ideas of Atoms
• Two Ancient Greeks (5th Century BC): Look at matter on smaller and smaller scales:
Ultimately you will see individual atoms - objects that cannot be divided further (the definition of atom or ).
• Galileo and Newton both believed in atoms
• Lavoisier, Dalton and Avogadro found support for atoms in Chemistry
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Billiard Ball Atoms• The theory of atoms only became widely
recognised when scientists could measure accurately the formulas of compounds.
• Knowing that water was formed from gases:• 2 volumes of hydrogen and 1 volume of
oxygen• suggested that H2O was the formula and
lead Dalton to hypothesise that water was made from 2 atoms of hydrogen joined to one atom of oxygen.
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Atomic Structure
• – 1808 state of atomic knowledge (Dalton)– Matter made of atoms like billiard balls– Atoms are electrically neutral– Atoms react in simple whole number ratios
e.g. H2O CO2 MgSO4
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Electrical Matter
• Davy and Faraday realised that matter is electrical in nature as a result of experiments involving electrolysis
• Now the billiard ball did not look so good
• Where were the electrical charges?
• More investigations were carried out to find out what was electrically charged
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Crookes 1875
• Crookes passed an electric current through an almost-evacuated tube.
• He found that a beam travelled from the cathode towards the anode and made the glass fluoresce
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JJ Thompson - 1897
• He found that the beam of cathode rays was deflected towards the positive electrode when passed through an electric field.
• The cathode rays must be:• Negatively charged.• He used the term “electron” (first used a
few years earlier) for these negative particles
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Thompson’s experiment:
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Plum Pudding• Thompson (Nobel Prize 1906) came up
with the plum pudding atomic model:
Uniform sphere of +ve charge with electrons embedded inside
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Goldstein - 1886
• Discovered that positive rays were emitted behind the cathode in the cathode ray tube.
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Positive Rays
• The mass of the particles in the positive rays depended on which element was in the Crookes tube
• The smallest positive rays were present when hydrogen was the initial gas.
• This smallest positive particle was eventually named the PROTON by Rutherford in 1914
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Becquerel- Radioactivity 1896
• One type of radiation discovered by Becquerel was Alpha particles
• These were found to be particles much smaller than atoms with a 2+ charge
• Rutherford and his co-workers decided to use a stream of alpha particles like bullets to probe the inner structure of atoms
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Rutherford Scattering
• Alpha particle scattering– Rutherford aimed alpha particles at
a thin foil – He expected all to go straight
through.– But noted that some were deflected
• Manchester 1909– Experiment performed just after
Thomson (Rutherford’s old boss) published his “Plum Pudding” paper
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Rutherford Scattering
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Geiger and Marsden
• Continued with Rutherford’s work
• Scattered alpha particles with heavy metal foils, particularly gold
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Geiger and Marsden
• Found that:
• Most alpha particles went straight through with very little deflection
• A few were deflected by large angles
• About 1 in 8000 was reflected
This image is taken from a Java Applet at: http://www.scri.fsu.edu/~jac/Java/rutherford.html
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Geiger Marsden Experiment
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Disproof of the Pudding
• Rutherford calculated from the results –1911:
• To reflect alpha particles the +ve charge (and most of the mass) has to be in a very small diameter
• About 1 x 10-15 m compared to 1 x 10-10 m for the diameter of the atom
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Disproof of the Pudding
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New Model:
• To explain the large size of the atom and the very dense nucleus the next model had both protons and electrons in the central nucleus and orbiting electrons.
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Chadwick 1932
• It took Chadwick’s discovery of the neutron to produce the more modern version of the atom:
• A nucleus containing protons and neutrons with electrons orbiting in shells.
• Even this is not the full story ………
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Solar System Model
• Positively charged nucleus at centre
• Negatively charged electrons in orbit
• Problem –– Orbiting electrons are accelerating –– Will give off energy –– Will spiral in to centre
• Model not stable
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Bohr Atom
• 1913 Bohr presented his theory (Nobel Prize 1922)
• Electrons in atoms can exist ONLY in certain discrete orbits, and they do not radiate energy
• When an electron jumps from one orbit to another its energy is exactly equal to the energy difference between the orbits
• Quantum theory was then the focus of research to explain the structure in more accurate detail
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Atomic Structure
• Atoms are approx 1 x 10-10 m in diameter
• Atoms consist of a positively charged nucleus surrounded by orbiting electrons
• The nucleus is approx 1 x 10-15 m in diameter
• Most of the atom is empty space
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Nuclear Structure
• The nucleus is made up of two particles (collectively called NUCLEONS)
• Protons and neutrons– Protons are +ve in charge– Neutrons are neutral
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Atomic Particles
Relative Charge Relative Mass
Electron -1 1/1840
Proton +1 1
Neutron No charge 1
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Describing the Nucleus• We describe the nucleus by noting its
Chemical Symbol along with– The number of protons Z– The number of nucleons A
• Each chemical symbol always has the same number of protons– Hydrogen – 1– Helium -- 4– Carbon – 6
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Describing the Nucleus
C126
indicates a lithium NUCLEUS with its usual 3 protons and a total of 7 nucleons (4 neutrons)
indicates a carbon NUCLEUS with its usual 6 protons and a total of 12 nucleons (6 neutrons)
Li73
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Isotopes
C146
C126
• These two represent isotopes of carbon– Chemically they would behave identically (if
they are combined with 6 electrons to make atoms)
– To a physicist they are different• Different masses• Different behaviour
– They can be separated by physics, not by chemistry