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.

every chemical equation adheres to the law of conservation of mass, which states that matter cannot be created or destroyed. therefore, there must be the same number of atoms of each element on each side of a chemical equation.