The formation of the oxide ion O2- (g), from the oxygen atom requires first an exothermic and then an endothermic step as shown below, 
 Og+e-O-; fH0=-141 kJ mol-1
O-g+e-O2-g; fH0=+780 kJ mol-1

Thus, the process of formation of O2- in the gas phase is unfavorable even though O2- is isoelectronic with neon. It is due to the fact that:

1. Electron repulsion outweighs the stability gained by achieving noble gas configuration.
2.   O- ion has a comparatively smaller size than the oxygen atom.
3. Oxygen is more electronegative.
4. Addition of electrons in oxygen results in a large size of the ion.
Subtopic:  Electron Affinity (EA) |
 75%
From NCERT
NEET - 2015
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The correct order of least negative to most negative electron gain enthalpy for C, Ca, Al, F and O is: 

1. Al < Ca < O < C < F
2. Al < O < C < Ca < F
3. C < F < O < Al < Ca
4. Ca < Al < C < O < F
Subtopic:  Electron Affinity (EA) |
 82%
From NCERT
NEET - 2013
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The value of electron gain enthalpy of Na+ if IE1 of Na = 5.1 eV ,is

1. + 10.2 eV 

2. –5.1 eV

3. –10.2 eV 

4. + 2.55 eV

Subtopic:  Electron Affinity (EA) |
 81%
From NCERT
AIPMT - 2011
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The correct order of increasing electron affinity for the elements, O, S, F and Cl is:

1. Cl < F < O < S 2. O < S < F < Cl
3. F < S < O < Cl 4. S < O < Cl < F
Subtopic:  Electron Affinity (EA) |
 77%
From NCERT
AIPMT - 2010
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Which one of the following arrangements does not give the correct picture of the trends indicated against it?
1. F2> Cl2> Br2> I2 : Oxidising power
2. F<Cl>Br>I : Electron gain enthalpy
3. F2> Cl2> Br2> I2 : Bond dissociation energy
4. F> Cl > Br > I : Electronegativity

Subtopic:  Ionization Energy (IE) | Electronegativity | Electron Affinity (EA) |
 67%
From NCERT
AIPMT - 2008
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