Define electronegativity

The ability of an atom to attract electrons.

Explanation:

Electronegativity is the ability of an atom to attract electrons.  In a bonded pair, electrons tend to stay closer to the more electronegative atom.  Atoms become more electronegative near the top right of the Periodic Table, with the highest electronegativity being Fluorine.

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.