Final answer:
Neon is most similar to Argon as both are noble gases with full valence electron shells, making them chemically inert compared to the reactivity of alkali metals, halogens, and chalcogens.
Step-by-step explanation:
The element Neon is most similar to Argon (Ar). Both Neon (Ne) and Argon (Ar) are members of the noble gases family, which is in group 18 of the periodic table. This group is known for having a complete valence electron shell, making the elements chemically inert. Neon and Argon have this in common, which is not shared by Lithium (Li), Fluorine (F), or Oxygen (O), which are in different families and have different chemical properties. For example, Lithium is in Group 1 and is an alkali metal, Fluorine is in Group 17 and is a halogen, and Oxygen is in Group 16 and is a chalcogen.
Lithium (Li), with an atomic number of 3, has just one electron in its outermost shell, making it highly reactive, unlike the noble gases. Noble gases, such as Neon and Argon, have their outermost electron shells completely filled, which explains their lack of reactivity and characteristic stability. Argon has a full third electron shell, while Neon has a full second electron shell, still, both are noble gases with full valence shells, making their chemical properties most similar when compared to the other elements listed.