Historical clue formulations

• Part of a nuclear family? — 2024-12-07
• Uranium 235, e.g. — 2013-10-24
• Member of a fictional Springfield baseball team — 2011-05-03
• Elemental form — 2008-04-24
• Heavy hydrogen, e.g. — 2005-12-26
• Elemental form — 2004-05-14
• Chemical relative — 2004-03-17
• Chemist's study — 2003-08-28
• Heavy hydrogen, e.g. — 2001-07-15
• U-235 or C-14, e.g. — 1999-02-16