Atomic Structure A level At The Sixth Form College Colchester.
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Transcript of Atomic Structure A level At The Sixth Form College Colchester.
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Atomic Structure
A level
AtThe Sixth Form College
Colchester
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Early Ideas of Atoms
• Two fifth-century BC Greeks, – Said look at matter on smaller and smaller
scales ultimately you would see individual atoms - objects that could not be divided further (the definition of atom).
• Galileo and Newton both believed in atoms
• Lavoisier, Dalton and Avogadro found support for atoms in Chemistry
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Atomic Structure
• 1900 – state of atomic knowledge– Matter made of atoms– Atoms are electrically neutral– Atoms contain -ve electrons -- which were
identified by JJ Thomson in 1897– Atoms must contain +ve charge
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Atomic Model
• Plum Pudding– Put forward by
Kelvin in 1903 and supported by JJ Thomson
– Uniform sphere of +ve charge with electrons embedded inside
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Rutherford Scattering
• Alpha particle scattering– Rutherford aimed alpha particles at
a thin foil and 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 while most alpha 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 the +ve charge (and most of the mass) has to be in a very small diameter
• About 10-15 m compared to 10-10 m for the diameter of the atom
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Disproof of the Pudding
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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 it emits a single photon, its energy is exactly equal to the energy difference between the orbits
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Atomic Structure
• Atoms are approx 10-10 m in diameter
• Atoms consist of a positively charged nucleus surrounded by orbiting electrons
• The nucleus is approx 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
Charge Mass
Electron -e
-1.6 x 10-19 C
9.11 x 10-31 kg
Proton +e
+1.6 x 10-19 C
1836 me
Neutron No charge 1839 me
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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 helium NUCLEUS with its usual 2 protons and a total of 4 nucleons (2 neutrons)
indicates a carbon NUCLEUS with its usual 6 protons and a total of 12 nucleons (6 neutrons)
He42
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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