Which are radioactive elements in periodic table?

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Not all elements are radioactive. Only elements having atomic number more than 83 are considered having radioactive nature (radio-isotopes). The elements with atomic number less than 83 are considered stable.

OLDER ANSWER:Theoretically they all are, it is just a matter of decay-speed(radioactivity).
In general, the elements with higher numbers (more protons and neutrons) are more unstable, and thus decay faster.
26 Iron is considered stable, but higher then 84 Polonium is considered active.
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In Science

Which elements in the periodic table are radioactive?

Tc - 43 - Technetium Pm - 61 - Promethium Po - 84 - Polonium At - 85 - Astatine Rn - 86 - Radon Fr - 87 - Francium Ra - 88 - Radium Ac - 89 - Actinium Th - 90 (MORE)

What are the radioactive element in the periodic table?

This is hard to answer. All elements have radioactive isotopes. Some elements have no known stable isotopes. It might be more instructive to put the periodic table aside (MORE)