1898 – France



‘1903 – Awarded the Nobel-Prize for Physics jointly with Marie and Pierre Curie’

picture of a rock displaying fluorescence under short wavelength radiation

The phenomenon of fluorescence – displayed under short wavelength radiation

Stimulated by WILHELM CONRAD ROENTGEN’s discovery of X-rays in 1895, Becquerel chanced upon the phenomenon that is now known as radioactivity in 1896. The Frenchman believed that Röntgen’s X-rays were responsible for the fluorescence displayed by some substances after being placed in sunlight. Although he was wrong to assume that fluorescence had anything to do with X-rays, he tested large numbers of fluorescent minerals.

He found that uranium, the heaviest element, caused an impression on a covered photographic plate, even after being kept in the dark for several days, and concluded that a phenomenon independent of sunlight induced luminescence.
Investigation isolated the uranium as the source of ‘radioactivity’, a name given to the occurrence by Mme. Curie.

The SI unit of radioactivity, the becquerel is named in his honour.

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    NIELS BOHR (1885-1962)

    1913 – Denmark

    ‘Electrons in atoms are restricted to certain orbits but they can move from one orbit to another’

    Bohr’s was the first quantum model for the internal structure of the atom.

    Bohr worked with RUTHERFORD in Manchester and improved upon Rutherford’s model, which said that electrons were free to orbit the nucleus at random.

    Classical physics insisted that electrons moving around the nucleus would eventually expire and collapse into the nucleus as they radiated energy. Bohr resolved the issue surrounding Rutherford’s atomic structure by applying the concept of quantum physics set out by MAX PLANCK in 1900.
    He suggested that the electrons would have to exist in one of a number of specific orbits, each being defined by specific levels of energy. From the perspective of quantum theory, electrons only existed in these fixed orbits where they did not radiate energy. The electrons could move to higher-level orbits if energy was added, or fall to lower ones if they gave out energy. The innermost orbit contains up to two electrons. The next may contain up to eight electrons. If an inner orbit is not full, an electron from an outer orbit can jump into it. Energy is released as light (a photon) when this happens. The energy that is released is a fixed amount, a quantum.

    Quanta of radiation would only ever be emitted as an atom made the transition between states and released energy. Electrons could not exist in between these definite steps. This quantised theory of the electrons’ orbits had the benefits of explaining why atoms always emitted or absorbed specific frequencies of electromagnetic radiation and of providing an understanding of why atoms are stable.

    Bohr calculated the amount of radiation emitted during these transitions using Planck’s constant. It fitted physical observations and made sense of the spectral lines of a hydrogen atom, observed when the electromagnetic radiation (caused by the vibrations of electrons) of the element was passed through a prism.
    The prism breaks it up into spectral lines, which show the intensities and frequencies of the radiation – and therefore the energy emissions and absorptions of the electrons.

    Each of the elements has an atomic number, starting with hydrogen, with an atomic number of one. The atomic number corresponds to the number of protons in the element’s atoms. Bohr had already shown that electrons inhabit fixed orbits around the nucleus of the atom.
    Atoms strive to have a full outer shell (allowed orbit), which gives a stable structure. They may share, give away or receive extra electrons to achieve stability. The way that atoms will form bonds with others, and the ease with which they will do it, is determined by the configuration of electrons.
    As elements are ordered in the periodic table by atomic number, it can be seen that their position in the table can be used to predict how they will react.

    In addition to showing that electrons are restricted to orbits, Bohr’s model also suggested that

    • the orbit closest to the nucleus is lowest in energy, with successively higher energies for more distant orbits.
    • when an electron jumps to a lower orbit it emits a photon.
    • when an electron absorbs energy, it jumps to a higher orbit.

    Bohr called the jump to another orbit a quantum leap.

    Although it contained elements of quantum theory, the Bohr model had its flaws. It ignored the wave character of the electron. Work by WERNER KARL HEISENBERG later tackled these weaknesses.

    Bohr’s theory of complementarity states that electrons may be both a wave and a particle, but that we can only experience them as one or the other at any given time. He showed that contradictory characteristics of an electron could be proved in separate experiments and none of the results can be accepted singly – we need to hold all the possibilities in mind at once. This requires a slight adjustment to the original model of atomic structure, we can no longer say that an electron occupies a particular orbit, but can only give the probability that it is there.

    In 1939 he developed a theory of nuclear fission with Jon Archibald Wheeler (b.1911) and realised that the 235uranium isotope would be more susceptible to fission than the more commonly used 238uranium.
    The element bohrium is named after him.

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    EDWARD TELLER (1908-2003)

    1952 – USA

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    In 1950 the H-Bomb project was begun in earnest, with Teller in a key role. Collaborative work between Teller and Stanislaw Marcin Ulam (1906-86) resulted in a thermonuclear device being ready by late 1951, with a public testing in 1952. This is a hydrogen-fusion device, as opposed to the atomic nuclear bomb. The latter works by essentially splitting the nucleus of the heavy, uranium atom; the former as an offshoot of forcing the conversion of hydrogen to helium. It was ENRICO FERMI (1901-54) who pointed out the possibility that an atomic explosion could cause enough heat and pressure to force a thermonuclear reaction of a hydrogen isotope, unleashing an even greater force.

    Scientific theory had hinted at this possibility ever since it was realised that a helium atom was slightly lighter than it should be given its component parts. An application of Einstein’s E=mc2 equation explained that the mass ‘lost’ in the fusion was being converted into huge amounts of energy, the basis upon which the Sun works, fusing hydrogen atoms into helium under great temperature and pressure and giving off the difference as radiation.

    Teller testified against ROBERT OPPENHEIMER during the investigation of alleged ‘disloyalty’.

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