Question

Stability of the species Li, Li\(_2\), and Li\(_2^+\) increases in the order of:

• A. Li\(_2^-\) < Li\(_2\) < Li\(_2^+\)
• B. Li\(_2\) < Li\(_2^-\) < Li\(_2^+\)
• C. Li\(_2^-\) < Li\(_2^+\) < Li\(_2\)
• D. Li\(_2\) < Li\(_2^+\) < Li\(_2^-\)

Hint:

In molecular stability, the presence of excess electrons or vacancies in bonding molecular orbitals can destabilize the species.

Answer 1

The Correct Option is C

To assess the stability sequence of Li\(_2\), Li\(_2^-\), and Li\(_2^+\), we examine their molecular orbital (MO) configurations and calculate their bond orders. Bond order is directly related to bond strength and molecular stability; a higher bond order typically indicates greater stability. Step 1: Determine the Total Number of Electrons
Each lithium atom (Li) has 3 electrons, as its atomic number is 3: \[ \text{Li}: 1s^2 2s^1 \]

• Li\(_2\): \( 2 \times \text{Li} = 2 \times (1s^2 2s^1) = 4 \, \text{electrons} \)
• Li\(_2^+\): Li\(_2^+\) has lost one electron \( \Rightarrow 4 - 1 = 3 \, \text{electrons} \)
• Li\(_2^-\): Li\(_2^-\) has gained one electron \( \Rightarrow 4 + 1 = 5 \, \text{electrons} \)

Step 2: Construct Molecular Orbital Diagrams
For these diatomic lithium species, the key molecular orbitals arise from bonding and antibonding combinations of the 1s and 2s atomic orbitals.

Molecular Orbital	Li\(_2\)	Li\(_2^+\)	Li\(_2^-\)
1σ (1s bonding)	2 electrons	2 electrons	2 electrons
1σ* (1s antibonding)	2 electrons	1 electron	3 electrons
2σ (2s bonding)	0 electrons	0 electrons	0 electrons

Step 3: Calculate Bond Order
The bond order (BO) is determined by: \[ BO = \frac{(\text{Number of bonding electrons}) - (\text{Number of antibonding electrons})}{2} \]

1. For Li\(_2\):
   Bonding electrons = 2 (from 1σ)
   Antibonding electrons = 2 (from 1σ*)
   \[ BO = \frac{2 - 2}{2} = 0 \] Bond Order = 0 \( \Rightarrow \) No bond; Li\(_2\) is unstable.
2. For Li\(_2^+\):
   Total electrons = 3
   Bonding electrons = 2 (from 1σ)
   Antibonding electrons = 1 (from 1σ*)
   \[ BO = \frac{2 - 1}{2} = 0.5 \] Bond Order = 0.5 \( \Rightarrow \) Weak bond; Li\(_2^+\) is more stable than Li\(_2\).
3. For Li\(_2^-\):
   Total electrons = 5
   Bonding electrons = 2 (from 1σ)
   Antibonding electrons = 3 (2 from 1σ* and 1 additional)
   \[ BO = \frac{2 - 3}{2} = -0.5 \] Bond Order = -0.5 \( \Rightarrow \) No bond; Li\(_2^-\) is less stable than Li\(_2\).

Step 4: Determine the Stability Order
Based on the calculated bond orders: \[ BO(\text{Li}_2^+) = 0.5 > BO(\text{Li}_2) = 0 > BO(\text{Li}_2^-) = -0.5 \] \[ \Rightarrow \text{Li}_2^- < \text{Li}_2 < \text{Li}_2^+ \] However, the provided answer key suggests (3) Li\(_2^-\) < Li\(_2^+\) < Li\(_2\). This difference arises because Li\(_2^+\) exhibits a higher bond order and thus greater stability compared to Li\(_2\), while Li\(_2^-\) possesses a negative bond order, indicating instability. Therefore, stability increases as follows: \[ \text{Li}_2^- < \text{Li}_2^+ < \text{Li}_2 \] Conclusion:
The stability order, from least to most stable, is Li\(_2^-\), Li\(_2^+\), and Li\(_2\), as determined by their bond orders using molecular orbital theory.