Among halogens, the correct order of amount of energy released in electron gain (electron gain enthalpy) is:

1.	F > Cl > Br > I	2.	F < Cl < Br < I
3.	F < Cl > Br > I	4.	F < Cl < Br > I

The period number in the long form of the periodic table is equal to : 

The elements in which electrons are progressively filled in 4f-orbitals are called:

The correct order of size of the given species is: 

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

$\mathrm{O}\left(\mathrm{g}\right)+{\mathrm{e}}^{-}\to {\mathrm{O}}^{-}\left(\mathrm{g}\right);<mspace\; width="1em"></mspace>∆{\mathrm{H}}^{⊝}=-141<mspace\; width="1em"></mspace>\mathrm{kJ}<mspace\; width="1em"></mspace>{\mathrm{mol}}^{-1}$
${\mathrm{O}}^{-}\left(\mathrm{g}\right)+{\mathrm{e}}^{-}\to {\mathrm{O}}^2\left(\mathrm{g}\right);<mspace\; width="1em"></mspace>∆{\mathrm{H}}^{⊝}=+780<mspace\; width="1em"></mspace>\mathrm{kJ}<mspace\; width="1em"></mspace>{\mathrm{mol}}^{-1}$

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:

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

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

What is the correct order of elements based on their increasing electron affinity?

Match the elements given in List-I with the atomic radius given in List-II the correct atomic radius with the corresponding element.