Here's why:

* Atomic Radius: Atomic radius refers to the distance between the nucleus of an atom and its outermost electron shell. Fluorine has a smaller atomic radius than lithium.

* Nuclear Charge: Fluorine has a higher nuclear charge (more protons in the nucleus) than lithium. This stronger attraction pulls the electrons closer to the nucleus, resulting in a smaller atomic radius.

* Electron Configuration: Both lithium and fluorine have only one electron shell. However, fluorine has more protons, making the nucleus more positively charged and pulling the electrons closer.

Therefore, fluorine's smaller atomic radius makes it the smaller atom.