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 species Ar, K+ and Ca2+ contain the same number of electrons. In which order do their radii increase?
1.  Ar < K< Ca2+
2. Ca2+ < Ar < K+
3. Ca2+ < K< Ar
4. K+ < Ar < Ca2+

Subtopic:  Atomic Size |
 82%
From NCERT
NEET - 2015
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The correct order of ionic radii is:

1. H->H+>H 
2. Na+>F->O2-
3. F->O2->Na+
4. N3->Mg2+>Al3+

Subtopic:  Atomic Size |
 80%
From NCERT
AIPMT - 2014
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Be2+ is isoelectronic with which of the following ions?
1. H+
2. Li+
3. Na+
4. Mg2+

Subtopic:  Electronic Configuration |
 91%
From NCERT
AIPMT - 2014
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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 the decreasing ionic radii among the following isoelectronic species is:

1. Ca2+>K+>S2->Cl-

2. Cl->S2->Ca2+>K+

3. S2->Cl->K+>Ca2+

4. K+>Ca2+>Cl->S2-

Subtopic:  Atomic Size |
 87%
From NCERT
AIPMT - 2010
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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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Among the elements Ca, Mg, P and Cl, the correct order of increasing atomic radii is:
1. Cl < P < Mg < Ca
2. P < Cl < Ca < Mg
3. Ca < Mg < P < Cl
4. Mg < Ca < Cl < P

Subtopic:  Atomic Size |
 78%
From NCERT
AIPMT - 2010
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Amongst the elements with the following electronic configurations, which one of them may have the highest ionisation energy?

1. [Ne] 3s2 3p3

2. [Ne] 3s2 3p2

3. [Ar] 3d10  4s2 4p3

4. [Ne] 3s2 3p1

Subtopic:  Ionization Energy (IE) | Electronic Configuration |
 74%
From NCERT
AIPMT - 2009
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