Stability of the species $\mathrm{Li}_{2}, \mathrm{Li}_{2}^{-}$ and $\mathrm{Li}_{2}^{+}$ increases in the order of:

(A) $\mathrm{Li}_{2}^{-}<\mathrm{Li}_{2}^{+}<\mathrm{Li}_{2}$

(B) $\mathrm{Li}_{2}<\mathrm{Li}_{2}^{-}<\mathrm{Li}_{2}^{+}$

(C) $\mathrm{Li}_{2}^{-}<\mathrm{Li}_{2}<\mathrm{Li}_{2}^{+}$

(D) $\mathrm{Li}_{2}<\mathrm{Li}_{2}^{+}<\mathrm{Li}_{2}^{-}$

JEE Main · 2013 · 07 Apr 2013 · Offline · Chemical Bonding and Molecular Structure

Stability of the species

Li_{2}, Li_{2}^{-}

and

Li_{2}^{+}

increases in the order of:

Answer: (A)

Molecular orbital filling gives the bond orders. Neutral

Li X_{2}

has bond order

\frac{1}{2} (4 - 2) = 1

; removing an electron for

Li X_{2} X^{+}

leaves

\frac{1}{2} (3 - 2) = 0.5

; adding one for

Li X_{2} X^{-}

gives

\frac{1}{2} (4 - 3) = 0.5

. The two ions tie on bond order, but

Li X_{2} X^{-}

carries its extra electron in an antibonding orbital, which makes it the least stable, so stability rises in the order

Li X_{2} X^{-} < Li X_{2} X^{+} < Li X_{2}

JEE Main · 2013 · 07 Apr 2013 · Offline · Single correct option