As a general rule for remembering which atoms undergo fission and fusion:
fusion fuses elements lighter than iron. fission divides elements heavier than iron.
fusion divides elements heavier than iron. fission fuses elements lighter than iron.
fusion fuses elements heavier than iron. fission divides elements lighter than iron.
fission and fusion reactions do not have any dependence on whether atoms are heavy or light.

Fusion fuses elements lighter than iron. Fission divides elements heavier than iron.

Explanation:

Fission is when an atom undergoes breaking into new elements and some sub atomic particles and energy.

Fusion is when two small nuclei fuses to form a new element of higher atomic number.

Thus the elements which are lighter than iron will undergo fusion to form new elements.

The elements which are heavier than iron may undergo fission and divide into smaller nuclei.

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