loach, misgurnus anguillicaudatu, Radium (element)
鐳 has two unrelated meanings: a type of loach (a fish) and the chemical element radium. There is no single conceptual center connecting them; treat them as separate senses.
鐳 is a modern character created for chemical element naming. It combines the metal radical 金 with the phonetic component 雷 (ライ), chosen for its sound to represent radium. The loach meaning is likely a separate, possibly older usage, but its historical development is uncertain.
The metal radical 金 hints at the element radium, a metal. The thunder component 雷 can remind you of the powerful energy of radioactivity. For the loach meaning, picture a loach hiding under a metal object during a thunderstorm.
For ライ, imagine a scientist shouting 'Radium!' as lightning strikes: the 'rai' in 'radium' cues ライ, and the thunder 雷 reinforces the sound.