Identify the least stable among the following

Options

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

Answer

B

Explanation

`Li(Z = 3) : 1s^(2) 2s^(1)`
`Li^(-) : 1s^(2) 2s^(2)`
Thus, `Li^(-)` is very stable. Electronic affinity of Li is `60 kJ mol^(-1)`.
`Be (Z = 4) : 1s^(2) 2s^(2)`
`Be^(-) : 1s^(2) 2s^(2) 2p^(1)`
Thus, `Be^(-)` will be highly unstable. Be has almost zero electron affinity.
`B(Z = 5) : 1s^(2) 2s^(2) 2p_(x)^(1)`
`B^(-) : 1s^(2) 2s^(2) 2p_(x)^(1) 2p_(y)^(1)`
Thus `B^(-)` will not be very unstable. Electron affinity of B is `29 kJ mol^(-1)`
`C(Z = 6) : 1s^(2)2s^(2) 2p_(x)^(1) 2p_(y)^(1)`
`C^(-) : 1s^(2) 2s^(2) 2p_(x)^(1) 2p_(y)^(1) 2p_(z)^(1)`
Thus, `C^(-)` is quite stable. Electron affinity of C is `12 kJ mol^(-1)`.
Thus most stable is `Li^(-)` and least stable is `Be^(-)`.