Final answer:
The bond order of Li₂ is predicted to be 1 using a molecular orbital energy-level diagram, indicating a stable bond. For Li₂⁻, the bond order is calculated as 0.5, showing that the bond exists but is weaker compared to Li₂.
Step-by-step explanation:
To predict the bond order of Li₂ and Li₂⁻, we need to use a qualitative molecular orbital (MO) energy-level diagram. Lithium, being in the second period of the periodic table, has its bonding and antibonding orbitals arranged in a certain way due to its electronic configuration. Specifically, the ordering of the σ and π orbitals is different for elements with three or fewer p electrons.
For Li₂, with each lithium atom contributing one valence electron, the MO energy-level diagram would show a total of two electrons. These electrons will fill the lower-energy bonding orbital (σ), and none would be in the antibonding orbital (σ*).
As such, the bond order for Li₂ would be calculated as (number of electrons in bonding orbitals - number of electrons in antibonding orbitals) / 2, which is (2-0)/2 = 1, indicating a stable bond.
For Li₂⁻, an additional electron is added to the next available orbital, which would be the antibonding orbital (σ*). This gives us a bond order of (2-1)/2 = 0.5. Although the bond order is less than that of Li₂, it still suggests a bond exists, albeit a weaker one compared to Li₂.