MgO Electron Affinity: Why is O +737 kJ?

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Discussion Overview

The discussion revolves around the electron affinity of oxygen in the context of magnesium oxide (MgO). Participants explore the thermodynamic processes involved in the formation of ions and the nature of electron affinity, particularly focusing on why the electron affinity of oxygen is reported as +737 kJ, contrasting it with other elements like fluorine.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant presents a series of thermodynamic equations related to the formation of MgO and questions the positive value of oxygen's electron affinity, suggesting it should be negative based on comparisons with fluorine.
  • Another participant states that the first electron affinity is exothermic, while subsequent affinities are always endothermic, but seeks clarification on the reasoning behind this pattern.
  • A different participant questions how an electron can bond to an already negatively charged species, noting that multiply charged ions like O2- are stable only in solid or liquid states due to surrounding positive ions.

Areas of Agreement / Disagreement

Participants express differing views on the nature of electron affinity, particularly regarding the positive value for oxygen and the behavior of electron affinities in general. The discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Participants highlight the complexity of electron affinity, including the dependence on the state of matter and the surrounding ionic environment, but do not resolve the underlying assumptions or definitions related to these concepts.

Who May Find This Useful

This discussion may be of interest to those studying thermodynamics, ionic bonding, or electron affinity in chemistry, particularly in the context of transition from gaseous to solid states.

osaka
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I have a problem with MgO(s).

Mg(s) → Mg(g) Heat of sublimation = +148 kj

½O2(g) → O(g) Bond dissociation = +249 kj

Mg(g) → Mg2+(g)+ 2e- IE1+IE2= +2201 kj

O(g)+2e- → O2- EA= +737 kj

look at the EA in peiodic table O(EA= -141 kj).
When O charged 2e- it seems to be like F(EA=-328 kj) charged 1e-.
it should be - not +.
but Why EA of O=+737kj ?
 
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the first electron affinity is exothermic and after always endothermic
 
thank you.
 
garytse86 said:
the first electron affinity is exothermic and after always endothermic

But, I would like to know why it is always exothermic and then always endothermic, I am trying to find the logic behind, but I cannot!
 
How do you bond an electron to a species that is already negatively charged?
Formally multiply charged ons like O^{2-} exist only in solids (or liquids) where they are surrounded by positive charged ions which provide a potential well to stabilize bonding of the seconde electron.
 
Oh Ok, Thank you very much!
 

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