To determine where the electron adds when N₂ changes to N₂⁻, we need to consider the Molecular Orbital (MO) configuration of N₂.

Step 1: Write the electronic configuration of N₂.
Nitrogen (N) has 7 electrons. So, N₂ has 14 electrons.
The MO configuration for molecules with 14 or fewer electrons (like N₂) is:
σ1s² σ*1s² σ2s² σ*2s² π2p⁴ σ2p² π*2p⁰ σ*2p⁰

Step 2: Identify the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO).
From the configuration above, the HOMO is σ2p.
The LUMO (the next available orbital for an incoming electron) is π*2p.

Step 3: Determine where the extra electron adds for N₂⁻.
When N₂ changes to N₂⁻, one electron is added to the N₂ molecule. This extra electron will occupy the LUMO, which is the π*2p orbital.

Option Analysis:
A) σ orbital: These are bonding orbitals and are already filled in N₂ (specifically σ2p is the HOMO, but the next available σ orbital is σ*2p, which is higher in energy than π*2p).
B) σ* orbital: The σ*2s orbital is filled. The σ*2p orbital is empty but higher in energy than π*2p.
C) π* orbital: This is the LUMO (π*2p) for N₂, so the incoming electron will occupy this orbital.
D) π orbital: These are bonding orbitals (π2p) and are already filled in N₂.

Correct Answer: (C)