The answer should be 1.
Atmoic number means the number of protons in an atom, (an atom contains protons, neutrons, and electrons) and the number of protons also equals to the number of electrons in an atom.
From that, we can know that Lithium has 3 electrons in total.
Since the 1st electron shell only allows 2 electrons, so the remaining 1 must be in the 2nd shell, which is also the out most shell since there's no more electrons.
Therefore, the answer is 1.
