Atoms and quantum phenomena. SASP 10 th June 2010. Decay. 4 natural radioactive decay series (starting with Thorium, Neptunium, Uranium, Actinium) spontaneous nuclear reactions: mother, daughter, radiation emitted (with time) all 4 series end at Pb (lead) [Pb is nice and stable]
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Atoms and quantum phenomena
SASP 10th June 2010
1932: neutron discovered by Chadwick
1934 onwards: experiments done by Fermi et al in Rome - neutron irradiation of elements, starting with lightest & working through the periodic table up to Uranium. Expected transuranic elements. Hahn, Strassman, Lise Meitner repeat with U.
with a critical mass of U, neutrons emitted give a ‘chain reaction’
small nuclei combine, releasing more energy than fission e.g.
Let’s look at some numbers and see if they make sense.
Mass of a proton = 1.673 x 10-27kg
Mass of a neutron = 1.675 x 10-27kg
Mass of a alpha particle (ppnn)
= 6.643 x 10-27kg
Before we go on, kg is not the most helpful unit here so we are going to use u, the atomic mass unit wherever possible. 1u = 1/12 the mass of a neutral 12C atom (i.e. including its six electrons)
1u = 1.66056 x 10-27kg
Mass of a proton = 1.007276u
Mass of a neutron =1.008665u
Mass of an electron = 0.000549u
Mass of Hydrogen atom = 1.007825u
Mass of Helium Atom = 4.002603
Mass of a alpha particle = 4.001505u
Classical physics could not explain
Einstein: ‘God does not play dice.’
Bohr: ‘Anyone who is not shocked by quantum theory has not fully understood it.’
Feynman: ‘things on a small scale behave nothing like things on a large scale.’ ‘I hope you can accept Nature as She is – absurd.’
Does the quantum world really exist, or is there only abstract quantum physical description?
There is a ‘ladder’ of energy levels in the atom.
Changes in electron arrangement correspond to the emission or absorption of a photon.
where h is the Planck constant,
This is what explains the line spectra that we have covered before. The colour palette could be imagined as a visual ‘map’ of the electron configuration for a specific element.
Atoms and molecules absorb & emit photons of characteristic energy or frequency, by changing their vibrational modes.
Electron leaves metal
highenergy violet quantum
λ = h/p = h/mv
Where λ is know as the de Broglie wavelength
The double-slit experiment with electrons
number of electrons
e) 140 000
(a 2-dimensional grating)