Answer: argon (Ar)
Ar, Z=18 => 1s2 2s2 2p6 3s2 3p6
The 3 level shell has 8 electrons. The element with one more electron, need to start a new shell to arrange for the last electron.
Argon is a noble gas. Its group in the periodic table is 18. All the noble gases have the last shell full, which conferes them a high stability and low reactivity.
