Final answer:
XeF2 is the molecule that cannot be formed without the central atom, xenon, exceeding an octet of electrons due to the presence of empty d orbitals allowing for an expanded valence shell.
Step-by-step explanation:
The species that cannot be formed unless the central atom exceeds an octet is XeF2. The central atom, xenon, is located in the third period of the periodic table, meaning it has access to empty d orbitals in its valence shell, allowing it to accommodate more than eight electrons. This goes beyond the traditional octet rule and is typical of expanded valence shell molecules. While compounds like PCl3 and NH3 follow the conventional octet rule, molecules such as XeF2 and SbCl5, which have central atoms from the third period or lower, often have more than eight electrons around the central atom, illustrating the concept of an expanded octet.
Some common examples of expanded octet molecules include PCl5, PF5, SF6, and XeF4, where these central atoms share more than four pairs of electrons with other atoms due to the availability of empty d orbitals for bonding. In the case of XeF2, xenon can share more than four pairs of electrons, accommodating up to twelve in its valence shell.