Page 12 - Radiochemistry and nuclear chemistry

CHAPTER 1

Origin of Nuclear Science

Contents

1.1. Radioactive elements 1
1.2. Radioactive decay 2
1.3. Discovery of isotopes 3
1.4. Atomic models 6
1.5. Literature 9

1.1. Radioactive dements

In 1895 W. Roentgen discovered that when cathode rays (i.e. electrons) struck the wall
of an evacuated glass tube, it caused the wall material to emit visible light (fluoresce), while
at the same time a very penetrating radiation was produced. The name X-ray was given to
this radiation. Learning about this, H. Becquerel, who had been interested in the fluorescent
spectra of minerals, immediately decided to investigate the possibility that the fluorescence
observed in some salts when exposed to sunlight also caused emission of X-rays. Crystals
of potassium uranyl sulfate were placed on top of photographic plates, which had been
wrapped in black paper, and the assembly was exposed to the sunlight. After development
of some of the photographic plates, Becquerel concluded (erroneously) from the presence
of black spots under the crystals that fluorescence in the crystals led to the emission of
X-rays, which penetrated the wrapping paper. However, Becquerel soon found that the
radiation causing the blackening was not "a transformation of solar energy" because it was
found to occur even with assemblies that had not been exposed to light; the uranyl salt
obviously produced radiation spontaneously. This radiation, which was first called uranium
rays (or Becquerel rays) but later termed radioactive radiation (or simply radioactivity) 1 ,
was similar to X-rays in that it ionized air, as observed through the discharge of
electroscopes.
Marie Curie subsequently showed that all uranium and thorium compounds produced
ionizing radiation independent of the chemical composition of the salts. This was
convincing evidence that the radiation was a property of the element uranium or thorium.
Moreover, she observed that some uranium minerals such as pitchblende produced more
ionizing radiation than pure uranium compounds. She wrote: "this phenomenon leads to the

1 The word radioactivity refers to the phenomenon per se as well as the intensity of the radiation observed.