The heaviest elements have the smallest radii because they are the most dense, due to the mass packed into a smaller atom.
Electronegativity decreases as the groups decrease because nonmetals are the most electronegative. Elements in higher groups have the most nonmetalic properties. Also, when an atom is larger, the valence electrons are farther away from the positive nucleus, which is why the larger atoms don't attract electrons as well.
The trends are referred to as being periodic, because they happen periodicly, just as magazines and newspapers come periodically (often).
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