MgO Electron Affinity: Why is O +737 kJ?

AI Thread Summary
The discussion centers on the complexities of electron affinity (EA) and ionization energy (IE) related to magnesium oxide (MgO). It highlights the heat of sublimation for magnesium and the bond dissociation energy for oxygen, leading to questions about the seemingly contradictory positive value of EA for oxygen when it gains two electrons. The conversation notes that while the first electron affinity of oxygen is exothermic, subsequent additions of electrons are endothermic, prompting inquiries into the underlying logic. It is explained that negatively charged ions like O²⁻ are stabilized in solid or liquid states by surrounding positive ions, which allows for the bonding of additional electrons. The discussion emphasizes the importance of understanding these energetic processes in the context of chemical bonding and stability.
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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