CK-12 Foundation
Ck 12: Physical Science: Radioisotopes
[Free Registration/Login may be required to access all resource tools.] What a radioisotope is and why they have unstable nuclei. Elements in which all of the isotopes are radioactive.
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Andy Darvill's Science Site: Types of Radioactivity
Find out about radioactivity, and how it occurs inside an atom's unstable nucleus.