Word: actinium
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Dates: during 1930-1939
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...probably) the triton. After them, in Nature's system of elements, comes helium (atomic weight approximately 4). The helium atom's nucleus is the alpha particle which, in the full round of substances, again appears during the disintegration of the heaviest of the 92 elements-uranium, thorium, actinium, radium...
...noteworthy that neutrons have been recognized as coming from elements which are the lightest of their groups in the Periodic System. Beryllium is the lightest of the earth alkali group (beryllium, magnesium, calcium, strontium, barium, radium). Boron is lightest of the earth metals (boron, aluminum, scandium, yttrium, lanthanum, actinium). Fluorine is lightest of the halogens (fluorine, chlorine, bromine, iodine and, newly recognized, alabamine...
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