Select the correct decay mode at each step to guide unstable nuclides to stability.
F-18 is injected into patients for PET scans. Its positron emission lets doctors image cancer and brain activity in real time.
Iodine-131 is used to destroy overactive thyroid tissue and treat thyroid cancer — the thyroid naturally concentrates iodine.
Polonium-210 poisoned Russian dissident Alexander Litvinenko in 2006. Just 1 μg delivers a lethal radiation dose.
Radon-222 seeps from uranium-bearing rocks into basements and is the second leading cause of lung cancer after smoking.
U-238's 4.5 billion year half-life matches Earth's age — roughly half of all primordial uranium has now decayed to lead.