The last group on the periodic table is called the noble gases. The elements in this group are all gases and are unreactive. Which property causes the noble gases to be unreactive?
A. atomic radius
B. atomic weight
C. full valence electron configuration
D. structure of the nucleus

C

Explanation:

They are stabil and full of valence electron.

since the atomic number is the number of protons in the nucleus, as we move to the left on the periodic table, the electrostatic pull between the nucleus and the valence electron shell decreases. as we move down a group (vertical column), the valence shell energy level gets further and further from the nucleus.