The formation of oxide ion O^2-(g), from oxygen atom, requires first an exothermic and then an endothermic step as shown below

$\mathrm{O}\left(\mathrm{g}\right)+{\mathrm{e}}^{-}\to {\mathrm{O}}^{-}\left(\mathrm{g}\right); ∆{\mathrm{H}}^{⊝}=-141 \mathrm{kJ} {\mathrm{mol}}^{-1}$
${\mathrm{O}}^{-}\left(\mathrm{g}\right)+{\mathrm{e}}^{-}\to {\mathrm{O}}^2\left(\mathrm{g}\right); ∆{\mathrm{H}}^{⊝}=+780 \mathrm{kJ} {\mathrm{mol}}^{-1}$

Thus, the process of formation of O^2- in the gas phase is unfavourable even though O^2- is isoelectronic with neon. It is due to the fact that

1.	Oxygen is more electronegative
2.	Addition of electron in oxygen results in a larger size of the ion
3.	Electron repulsion outweighs the stability gained by achieving noble gas configuration
4.	O- ion has comparatively smaller size than oxygen atom

The outermost electronic configuration of the last element of the p-block in the 6th period is represented by:

1. $7{\mathrm{s}}^{2}7{\mathrm{p}}^6$

2. $5{\mathrm{f}}^{14}6{\mathrm{d}}^{10}7{\mathrm{s}}^{2}7{\mathrm{p}}^0$

3. $4{\mathrm{f}}^{14}5{\mathrm{d}}^{10}6{\mathrm{s}}^{2}6{\mathrm{p}}^6$

4. $4{\mathrm{f}}^{14}5{\mathrm{d}}^{10}6{\mathrm{s}}^{2}6{\mathrm{p}}^4$

Among the following elements whose atomic numbers are given below, which cannot be accommodated in the present set up of the long periodic table?

The elements with atomic numbers 35, 53, and 85 are:

Electronic configuration of four elements A, B, C and D are given below : 

A. $1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^6$

B. $1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^4$

C. $1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^1$

D. $1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^5$

The correct order of increasing tendency to gain electron is : 

Match the correct atomic radius with the corresponding element.

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Match the correct ionization enthalpies and electron gain enthalpies of the following elements.

Codes

Electronic configuration of some elements is given in Column I and their electron gain enthalpies are given in Column II. Match the electronic configuration with electron gain enthalpy.

Codes

Given below are two statements: 

Given below are two statements: