Word: actinium
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...close relative of uranium, thorium is another radioactive, heavy element (atomic number 90, atomic weight 232.12) that disintegrates into lower-weight elements and eventually becomes lead. Of the three standard radioactive progressions - uranium-radium, actinium and thorium-those of uranium and thorium are the most alike. According to the Smyth report, thorium was considered as a basic source of atomic power, but uranium was chosen instead...
...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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