Related Clues:

• __ bomb
• Hard thing to split
• Elemental bit
• Charged subject in science class?
• Splitting it releases energy
• What a plus sign may indicate
• Molecule in some ostensibly healthier water
• Na+, e.g.
• Excitable one
• Mr. ___, radioactive enemy of Captain Marvel
• Atom with extra electrons, perhaps
• Matter constituent
• Concern for Edward Teller
• ___ drive (engine in 'Star Wars')
• One created by hydrolysis