To determine the stability order of Li₂, Li₂^-, and Li₂⁺, we use Molecular Orbital Theory (MOT). The stability of a diatomic species depends on its bond order: higher bond order means greater stability and bond strength.

Step 1: Write the molecular orbital configuration for Li₂
Lithium (Li) has atomic number 3, so its electron configuration is 1s² 2s¹. For Li₂, we consider valence electrons (2s orbitals). The molecular orbital energy order for Li₂ is σ_2s < σ*_2s.

Li₂ has 6 electrons total, but 4 are in 1s (core orbitals, not involved in bonding). Valence electrons = 2.
Configuration: (σ_2s)²
Bond order = (Number of bonding electrons - Number of antibonding electrons)/2 = (2 - 0)/2 = 1

Step 2: For Li₂⁺
It has one less electron than Li₂, so valence electrons = 1.
Configuration: (σ_2s)¹
Bond order = (1 - 0)/2 = 0.5

Step 3: For Li₂^-
It has one extra electron, so valence electrons = 3.
Configuration: (σ_2s)² (σ*_2s)¹
Bond order = (2 - 1)/2 = 0.5

Step 4: Compare bond orders
Li₂: bond order = 1
Li₂⁺: bond order = 0.5
Li₂^-: bond order = 0.5

Both Li₂⁺ and Li₂^- have the same bond order (0.5), but Li₂^- has an extra electron in the antibonding orbital, which adds repulsion and makes it less stable than Li₂⁺. Also, Li₂⁺ has fewer electrons, reducing electron-electron repulsion. Thus, stability order is Li₂^- < Li₂⁺ < Li₂.

Final Answer: The stability increases in the order: Li₂^- < Li₂⁺ < Li₂

Related Topics

Molecular Orbital Theory (MOT): MOT describes the electronic structure of molecules using molecular orbitals formed by the combination of atomic orbitals. Bond order is a key concept, calculated as (bonding electrons - antibonding electrons)/2. Higher bond order indicates stronger and more stable bonds.

Bond Order and Stability: Bond order directly correlates with bond strength and stability. It also influences bond length and magnetic properties.

Formulae

Bond Order = $\frac{N_b-N_a}{2}$, where $N_b$ is number of bonding electrons and $N_a$ is number of antibonding electrons.