Which element has a complete valence electron shell?
selenium (se)
oxygen (o)
fluorine (f)
argon (ar)

The element that has a complete valence electron shell is

Argon(Ar)

Argon is in group 18 and period three in the periodic table. Therefore Argon electronic configuration is 2.8.8 or 1s^2 2s^2 2P^6 3S^2 3P^6 or [Ne] 3s^2 3p^6. The valence electrons of Argon is 3s^2 3p^6 which are completely filled since an 3S subshell can hold a maximum of 3 electrons while a 3P subshell can accommodate a a maximum of 6 electrons hence, Argon has a complete filled valence shell.

the electron configuration that you gave me has 16 electrons ergo it has 16 protons ergo it is the element sulfur.

it forms an anion that has a larger radius. anions have a negative charge and are larger than the original atom.