The least stable in amongst the following is :

Options

A. `C^(-)`
B. `B^(-)`
C. `Be^(-)`
D. `Li^(-)`

Answer

C

Explanation

`Li(1s^(2)2s^(1))rarrLi^(-)(1s^(2)2s^(2))`
`Be(1s^(2)2s^(2))rarr Be^(-) (1s^(2)2s^(2)2p^(1))`
`B(1s^(2)2s^(2)1p^(1)) rarr B^(-)(1s^(2)2s^(2)2p_(x)^(1)2p_(y)^(1))`
`C^(-)(1s^(2)2s^(2)2p^(2))rarr C^(-) (1s^(2)2s^(2)2p^(3))`
`Be^(-)` is the least stable because in the Be atom, all the subshells are completely full. Thus, on addition of `1` electron, the stable configuration is destabilized.