Identify the least stable ion amongst the following.

{L i}^{-}

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{B e}^{-}

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B^{-}

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C^{-}

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Solution

The correct option is B ${B e}^{-}$
${B e}^{-}$ is the least stable ion among all. ${L i}^{-} : {1 s}^{2}, {2 s}^{2}; {B e}^{-} : {1 s}^{2}, {2 s}^{2}, {2 p}^{1} .$

In $L i$ addition of electron has taken place in $2 s$ orbital. In ${B e}^{-}$ , addition of electron has taken place in $2 p$ orbital loosing its $2 s$ completely filled configuration.

${E A}_{1}$ for $B e$ is more positive than ${E A}_{1}$ for $L i$ . Thus ${B e}^{-}$ is least stable.

$B^{-} : {1 s}^{2}, {2 s}^{2}, 2 p^{2}$ where electrons can be easily added to make it half filled and stable. $C^{-} : {1 s}^{2}, {2 s}^{2}, 2 p^{3}$ , the valence orbital is half filled and stable.

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