Identify the least stable ion amongst the following:
(1) Li-
(2) Be-
(3) B-
(4) C-

Answer: 2
Explanation:
1. Li-: Lithium has an atomic number of 3, and when it gains one electron, it forms Li-, which has a stable electronic configuration of 1s2 2s2.

2. Be-: Beryllium has an atomic number of 4, and when it gains one electron, it forms Be-, which has an unstable electronic configuration of 1s2 2s2 2p1. Adding one electron will disturb its stable fully filled configuration. So, it is the least stable among all.

3. B-: Boron has an atomic number of 5, and when it gains one electron, It does not have half-filled or fully filled configuration so adding one e- will not disturb its stable configuration.

4. C: Carbon has an atomic number of 6, has a stable electronic configuration of 1s2 2s2 2p2 and when it gains one electron, it forms C-, which has a stable half-filled electronic configuration of 1s2 2s2 2p3. Therefore, the correct answer is (2) Be-.

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