1

### JEE Main 2018 (Online) 15th April Evening Slot

The correct order of electron affinity is :
A
F > Cl > O
B
F > O > Cl
C
Cl > F > O
D
O > F > Cl

## Explanation

Electron affinity means tendency of gaining an electron by an atom.

In a period from left ot right the electron affinity increases and in a group it decreases from top to bottom.

So according to this theory Fluorine(F) should have most electron affinity. But when an electron is added to the F atom, electron comes to the 2p orbital and for Cl atom electron is added in 3p orbital. As 2p orbital is closer to the nucleus than 3p orbital as 3p orbital is larger in size, so when a new electron comes to 2p orbital then it will face a strong repulsion force by the nucleus than if electron comes to 3p orbital.

So, F have lesser tendency of gaining electron than Cl.

Note : In entire periodic table Cl have highest electron affinity.
2

### JEE Main 2019 (Online) 9th January Morning Slot

Aluminium is usually found in +3 oxidation state. In contrast, thallium exists in + 1 and + 3 oxidation states. This is due to :
A
inert pair effect
B
diagonal relationship
C
lattice structure
D
lanthanoid contraction

## Explanation

Electronic configuration of Thallium (Tl) is
= [Xe]4f14 5d10 6s2 6p1

Here because of poor sheilding of 4f and 5d orbital on 6s orbital, electrons of 6s orbital is more tightly held by the nucleus and perticipate less in bond formation. This is called as innert pair effect. And that is why Tl become stable in +1 oxidation state.
3

### JEE Main 2019 (Online) 9th January Morning Slot

In general, the properties that decrease and increase down a group in the periodic table, respectively, are :
A
B
Electron Gain Enthalpy and Electronegativity.
C
D
Electronegativity and Electron Gain Enthalpy.

## Explanation

Electronegativity decreases down the group because the increased number of energy levels puts the outer electrons very far away from the pull of nucleus.

Atomic radius increases down the group because the number of energy levels increases when you move down the group. Each subsequent energy level is further from the molecules than the last. That is why atomic radius increases down the group.
4

### JEE Main 2019 (Online) 9th January Evening Slot

When the first electron gain enthalpy $\left( {{\Delta _{eg}}H} \right)$ of oxygen is $-$ 141 kJ/mol, its second electron gain enthalpy is :
A
a more negative value than the first
B
almost the same as that of the first
C
negative, but less negative than the first
D
a positive value

## Explanation

Second electron gain enthalpy is always positive for every element.
O$-$(g) + e$-$ $\to$ O$-$2(g) ; $\Delta$H = positive