Atoms of the same element are alike because they must have the same number of

They must have the same number of protons.

Explanation:

Protons determine the identity of an element. However, the neutrons can vary, resulting in different masses.

Atoms of the same element can have different numbers of neutrons, however. Atoms of the same element (i.e., atoms with the same number of protons) with different numbers of neutrons are called isotopes. Most naturally occurring elements exist as isotopes.

Explanation:

nuclear fission produces elements that are heavier than helium.

the elements that are used in nuclear fission and their products are much heavier than helium.