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₂He⁴  

Explanation:

Ra-266 undergoes alpha decay and emit helium nuclei.

Alpha radiations are emitted as a result of radioactive decay. The atom emit the alpha particles consist of two proton and two neutrons. Which is also called helium nuclei. When atom undergoes the alpha emission the original atom convert into the atom having mass number less than 4  and atomic number less than 2 as compared to parent atom the starting atom.

Properties of alpha radiation:

Alpha radiations can travel in a short distance.

These radiations can not penetrate into the skin or clothes.

These radiations can be harmful for the human if these are inhaled.

These radiations can be stopped by a piece of paper.

₈₈Ra²²⁶   →   ₈₆Rn²²²  + ₂He⁴