| fermium | <chemical> Fermium. A man-made radioactive actinide with atomic symbol fm, atomic number 100, and atomic weight 257. Its known isotopes range from 244-254 and 256-258. Its valence can be +2 or +3. Like einsteinium, it was discovered in 1952 in the debris from a thermonuclear explosion. Chemical name: Fermium (12 Dec 1998) |
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