The atom with 8 valence electrons (a full valence shell) is the least likely to form a chemical bond compared to the others.
Atoms with a full valence shell, such as the noble gases (Group 18 elements), have stable electron configurations and tend to be unreactive. This is due to the octet rule, which states that atoms typically gain, lose, or share electrons to achieve a stable configuration with 8 electrons in their outermost shell (except for hydrogen and helium, which can achieve stability with 2 electrons).
Therefore, an atom with 8 valence electrons is less likely to form a chemical bond because it already has a stable configuration, and gaining or losing electrons would require significant energy input.