Question

Identify the least stable ion among the following:

• A. Li^-
• B. Be^-
• C. B^-
• D. C^-

Answer 1

The correct option is B Be^-

Outer electronic configuration of the ions are $L{i}^{-}$ ($2s^2$ most stable), $B{e}^{-}$ ($2s^2$ $2p^1$ least stable), $B^{-}$ ( $2s^2$ $2p^2$ stable) and $C^{-}$ ( $2s^2$ $2p^3$ more stable because of the half-filled configuration).

As $B{e}^{-}$ is obtained from most stable Be ($2s^2$ configuration) by adding one electron, it is least stable and always has the tendency to lose one electron to achieve $2s^2$ configuration.