Answer: If its outermost electron shell is filled.
Explanation:
When an atom has deficiency or excess of electrons then in order to attain stability the atom will react readily with another atom. Whereas an atom whose outermost electron shell is completely filled is least likely to bond with other atoms as the atom is stable due to completely filled shell. Thus, we can conclude that a case in which if atom's outermost electron shell is filled then it is least likely to bond with other atoms.
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