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So Chadwick had proved the existence of yet another fundamental particle, and completed the set of fundamental sub-atomic particles – a positively charged nucleus containing protons (positive charge) and neutrons (neutral charge) of approximately the same mass (neutrons with a 0.1 percent increase on the mass of a proton), surrounded by what was then assumed to be an orbital collection of electrons of a discrete quantity (i. e. distinct from Thomson’s plum-pudding theory).

This was the state of atomic theory as left by Chadwick in 1932, who’s work eventually led to the use of neutron bombardment for the fission of uranium 235 to release the huge amounts of energy predicted by Einstein’s equation E=mc2 in the interests of power production and nuclear weapons.

The neutrality of the neutron provided physicists with a useful tool to overcome the enormous electrical barrier surrounding some of the heavier atoms, which could penetrate and split the nuclei of the heaviest of elements. Neils Bohr succeeded in constructing a theory of atomic structure based on quantum principles, and went some way to describing the mechanics of electron distribution throughout the electron shells. Much of Bohr’s work was based on the behaviour of hydrogen, which is the simplest element to work with in many respects.

The crucial question to which Bohr sought an answer was how does an electron orbit the nucleus and not lose energy? According to classical physics, a charged body moving in a circle is accelerating and therefore radiating energy – an electron would lose energy and crash into the nucleus in less than 1. 0 x 10-11 (0. 00000000001) seconds. In order to account for this peculiarity which invalidated understandings of atomic stability, Bohr introduced concepts borrowed from quantum theory based on Plank’s equations, from which the branch of theoretical physics had stemmed.

It was Plank who first proposed – in direct contradiction of classical physics – that the energy levels of electrons could not change continuously, but instead were restricted to certain discrete values, or quanta. Effectively this suggested that only defined orbits with distinct radii were feasible, that alternative orbits simply did not exist. The magnitude of the quantum was found by Plank to be proportional the oscillation of a particle, which was theoretically the frequency of the radiation emitted by that same particle.

This proportionality was named Plank’s constant, and while his original suppositions were intended to be purely theoretical in nature and more mathematically based than really applicable to aspects of physics, his equations and findings stood the test of time and found many applications. It was this idea of quantum levels of energy which led Bohr to the discovery of the major laws concerning atomic stability and spectral lines – to account for the line spectrum of hydrogen and atomic stability Bohr had applied Plank’s theories.

Bohr postulated that there existed a lowest energy level or “ground state” – quantum number (n) = 1, that was reached by electrons when all possible quanta of energy had been depleted. That when an atom absorbs energy it undergoes excitation and the energy level, or quantum number (n) increases. That when it loses energy it undergoes de-excitation, and n decreases. Electrons had to change their energy levels by jumping between the stationary states as they underwent excitation and de-excitation, emitting light (photons), the wavelength of which depended on the energy difference between levels; the quantum number.

This postulate was even more radical than Plank’s original equations and suggestions, but it was highly successful in providing a quantitative explanation for the hydrogen spectra, and solved the problem of atomic stability by allowing for a ground state. Although it proved quite obviously to be full of contradictions and inconsistencies, it was still vastly successful in opening the doors to quantum physics, as Thomson had opened the doors to the study of atomic particles, as Chadwick had opened the doors to the study of nuclear physics.

Bohr made other major contributions in addition to quantum theory, including his description of the periodic table around 1920 based on his theoretical line spectra concept, the Bohr model of the atom (1936) which consisted of a compound nucleus (Rutherford’s proton and Chadwick’s neutron) surrounded by a non-planetary orbital system of quantized electrons, and significant contributions to the development of nuclear power and the atomic bomb through his development and refinement of the liquid drop model of fission to be applicable to heavy elements such as the uranium that was used during the Manhattan project (of which Bohr was a part).

It could be argued that the nature of Bohr and Chadwick’s contributions to physics was different to that of Thomson’s – who’s discovery was so fundamental to the basis of classical physics atomic theory, but yet so radical. Similarly, it could be put forward that Thomson and Bohr were most alike in the nature of their discoveries in that both proposed extremely radical theories and launched physics into new eras.

It could however also be argued that Thomson and Chadwick belonged to a separate era of largely classical physics not reliant on quantum theories, and that Bohr’s contributions were largely based on a totally radical and separate “version” of physics to classical physics. Essentially, these three scientist were all alike in one important respect: each contributed essential elements of atomic theory to propel the study of physics down new avenue’s of research.

Bohr did for quantum theory what Chadwick did for nuclear physics, and what Thomson did for the study of atomic particles. Chris Bolton Acknowledgments: Macmillan Encyclopedia 1994 Edition Internet: http://www. nobel. se/physics/laureates/1906/thomson-lecture. pdf [28-01-04] http://www. nobel. se/physics/laureates/1935/chadwick-lecture. pdf [28-01-04] http://www. nobel. se/physics/laureates/1922/bohr-lecture. pdf [28-01-04] http://onsager. bd. psu. edu/~jircitano/neutron. html [28-01-04] http://library. thinkquest.

org/C0110925/html/index. html [28-01-04] http://www. 4physics. com:8080/phy_demo/QM_Article/article. html [28-01-04] http://dbhs. wvusd. k12. ca. us/Chem-History/Bohr-Fission-1939. html [28-01-04] http://particleadventure. org/particleadventure/other/history/quantumt. html [28-01-04] http://library. thinkquest. org/C0110925/html/theatom/atomicstructure/atomicstructure13. html [28-01-04] http://library. thinkquest. org/C0110925/html/history/disciplesoftheatom/disciplesoftheatom1. html [28-01-04] Test the mass of a drop of water

Force it to undergo liquid drop fission Test the mass of each separate droplet Neils proposed that a fissionable nucleus could be manipulated in a similar fashion to produce large quantities of energy. It was not long before the generative potential of a chain reaction of the fission of heavy isotopes was realised and utilised in the development of the atomic bomb in the Manhattan project (of which Neils was a part). liquid drop model of fission: nucleons interact strongly with each other as the molecules in a drop of water do.

Sufficiently deform the droplet by elongation, and the surface energy decreases (due to increased surface area). Then either the droplet will return to its original spherical shape, or coulomb repulsion will take place and the droplet will split into two separate spheres and be repulsed just outside the range of surface forces. The former case occurs when the charge density of the droplet is small compared to the surface energy.

If the original droplet is highly charged (i. e. with a large proportion of charge density against surface energy (SURFACE TENSION ONLY???)) then minute displacements of original spherical shape will result in violent coulomb repulsion exceeding the magnitude of surface energy and resulting in abrupt splitting of the drop of water THOMSON and the e/m of his corpuscles remained constant even when the electrodes in the cathode tube were changed, and even when the experiment took place in the presence of a variation of gases.

He discovered a method for separating different kinds of atoms and molecules by the use of positive rays, an idea developed by Aston, Dempster and others towards the discovery of many isotopes.

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