Question

Electron affinity is positive when____.
A. ${O^ - }$ is formed from O.
B. ${O^{2 - }}$ is formed from ${O^ - }$
C. ${O^ + }$ is formed from O.
D. Electron affinity is always a negative value.

Answer 1

Hint: Electron affinity is the change in energy due to the addition of electrons in the neutral atom. High energy is required to add extra electrons to the resulting negative ion. The energy thus needed is a lot more than the energy which is gained.

Complete answer:
The change in energy of a neutral atom present in a gaseous state, when an electron from the surrounding atom is added to the neutral atom, generates a negative ion called electron affinity. The change in energy is calculated in terms of KJ/mole.
Energy of an atom is determined when it gains or loses electrons during the chemical reaction. When an atom loses its electron it forms a positive charge ion known as cation and when atom gains its electron it forms a negative charge ion known as anion.
When in a neutral atom, the electron is added then the atom becomes negative. The energy needed to form a negative ion is the first electron affinity. For adding more electrons to the negative ion, extra energy is needed. This energy is the second electron affinity. In the second electron affinity, the energy released is positive as the high energy is required than the gain.
First electron affinity: $X(g) + {e^ - } \to {X^ - }(g)$
Second electron affinity: ${X^ - }(g) + {e^ - } \to {X^{2 - }}(g)$
Thus, when ${O^{2 - }}$ is formed from ${O^ - }$the electron affinity is positive.
Therefore, the correct option is B.

Note: When the energy is released, then the reaction is said to be an exothermic reaction and the sign of the reaction is negative and when the energy is absorbed, then the reaction is said to be an endothermic reaction and the sign of the reaction is positive.