Question

why Neon atomic size is greater than Fluorine?

Answer 1

because repulsion between the different shells of electron thus increase the size so much that it is even bigger than first element in period thats why neon atomic size is grater than fluorine,
and it is also because of structural stability of neon.

Answer 2

Due to completely filled orbitals in the atom of neon,a repulsion between the shells and the nucleus occur thereby increasing the atomic size .So,its atomic size would be greater than fluorine though the atomic size should decrease on moving from left to right in a period