Which nuclear equation represents a natural transmutation?

I can tell you about:

- What natura transmutation is

- Which elements suffer natural transmutation

- Some examples of nuclear equation representing natural transmutation

1) Natural transmutation is the transmutation that occurs in nature without intervention of human beings. So, the natural transmutation is the natural decay of elements into other (smaller) elements. The transmutation of the elements consists in the emission of particles (alpha emission, beta emission, positron emission or electron emission) and the obtention of a smaller element.

2) Which elements undergo natural transmutation.

The heavier elements, after lead, are radioactive and so they decay into lighter (smaller) elements. Those elements are unstable due to the relation proton / neutron.   

3) Transmutation equation examples

I131 > Xe131 + Beta particle

Ra222 ---> Rn218 + 2He4 (same alpha particle)

90Th234 > 89Pa234 + beta particle
 

92U238 > 90Th 234 + 2He4   (same alpha particle)

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