Nuclear

Part 13 - Isotopes

1509 Isotopes (690) 210615

Francis Aston's invention of the mass spectrometer in 1919 allowed him to identify 212 of the 281 naturally occurring isotopes, proving, for example, that neon consisted mostly of two stable isotopes: 20 Ne (90.48%), and 22 Ne (9.25%) which has two extra neutrons in the nucleus. (there is also a third isotope, 21 Ne (0.27%)) The two main isotopes 20 Ne and 22 Ne exist in the ratio of about 9 to 1 so that the average atomic weight is about 20.2.   This explained why atomic weights were not all exactly multiples of one.

The atomic weights of all isotopes were very nearly whole numbers with the odd exception of hydrogen (the most common isotope having one proton) at 1.008. But why was the atomic weight of helium (with 2 proton and 2 neutrons) exactly 4.000 and not 4.032?

 Aston reasoned that the missing mass was caused by the energy required to bind the protons and neutrons together. Without the neutrons the positively charged protons would repel each other and the nucleus could not exist. If four hydrogen atoms could be combined into one helium atom, this would release an enormous amount of energy (as produced in the Sun) according to Einstein's famous formula (Energy = mass times the velocity of light squared).Aston noted that elements in the middle of the periodic table were more stable than those at both ends, implying that the heaviest atoms would release energy if they could be broken down.

Deuterium, the heavy isotope of hydrogen having a nucleus with one proton and one neutron, had not been found before 1931 because of its rarity  (only about 1 atom in 6400 hydrogen atoms in ocean water).

Harold Clayton Urey, an American physical chemist, first detected deuterium spectroscopically in late 1931 using 1 mL of liquid hydrogen, that had been cryogenically concentrated with deuterium, from 5 L of naturally occurring hydrogen by Ferdinand Brickwedde. The discovery of deuterium earned Urey the Nobel Prize in Chemistry in 1934.

In 1933 Leo Szilard, remembering H.G. Wells' book about atomic bombs, The World Set Free, decided to find the element which could be split and produce a chain reaction.

In 1934, Enrico Fermi, in Italy, set out to bombard every known element with protons from a radon/beryllium source. He was able to induce radio-activity in aluminum which had a half-life of twelve minutes and later with twenty other elements from iron to lanthanum. When the team bombarded uranium (atomic number 92), they found it transmuted into a new heavier element with the atomic number of 93 (Later named plutonium). When it spontaneously fissioned, it produced a bewildering sequence of decay products indicated by their various half-lives.

Rutherford approved. He hypothesized that the 13-minute and 100-minute half lives were the result of transuranic elements. The 15-second, 13 minute and 100-minute activities were probably chain products as one decayed to the next. The elements involved were probably atomic number 92 (isotopes of uranium), 93 and 94 with atomic weight 239.

The German chemist, Ida Noddack objected, pointing out that when heavy nuclei were bombarded with neutrons, it was conceivable that the nucleus would break up into fragments of much lighter elements much farther down the periodic table. 

But Fermi refused to believe that uranium could be split in two. He checked Noddack's calculations, got a different result and forgot about it.

When Fermi's team attempted to refine their measurements, they got different results depending on the location of the experiment in the laboratory. Silver became more radio-active when bombarded on a wooden table than a nearby marble table. Fermi, puzzled, discovered that the induced radiation was considerably stronger when a thin piece of paraffin wax was placed in the neutron beam.

Testing a range of materials, he concluded that when the neutrons collided with hydrogen nuclei in the wax (or the wood) they were slowed down (moderated) and this increased the probability that they would remain near the target nuclei longer and be absorbed, rather than simply travelling completely through the target at high speed.