JEE Main · 2019 · 09 Jan 2019 · Shift 1 · Chemical Bonding and Molecular Structure

According to molecular orbital theory, which of the following is true with respect to

Li_{2}^{+}

and

Li_{2}^{-}

Answer: (B)

Solution

Build the molecular orbitals.

Li X_{2} X^{+}

has five electrons,

(σ_{1 s})^{2} (σ_{1 s}^{*})^{2} (σ_{2 s})^{1}

, bond order

\frac{3 - 2}{2} = 0.5

Li X_{2} X^{-}

has seven, adding one electron into the antibonding

σ_{2 s}^{*}

, also bond order

\frac{4 - 3}{2} = 0.5

. Although the bond orders match, the cation carries fewer antibonding electrons and is the more stable species, so

Li X_{2} X^{+}

is stable while

Li X_{2} X^{-}

is comparatively unstable.

JEE Main · 2019 · 09 Jan 2019 · Shift 1 · Single correct option