ELECTRON AFFINITY MCQ
IIT JEE - NEET

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1) The electron affinity values (in kJ mol^-1) of three halogens X, Y and Z are respectively -349, -333 and -325. Then X, Y and Z respectively are:

(EAMCET 2003-M)

1) F, Cl and Br 

2) Cl, F and Br 

3) Cl, Br and F 

4) Br, Cl and F

Logic:

Electron affinity is the measure of nuclear attraction over the newly added electron. It is the energy liberated when one electron is added to gaseous isolated atom. These values are represented with a negative sign. Greater the negative value greater is the electron affinity.

In smaller atoms, the newly added electron is attracted strongly by the nucleus and hence their electron affinity values are higher. In general, these values increase with decrease in the size.

However, in too small atoms, the new electron not only experiences the attraction from the nucleus but also the repulsions from the core electrons, which may diminish the electron affinity.

Solution:

In halogens, the electron affinity increases from F to Cl and then decreases to I as the atomic size increases. F has less electron affinity than Cl due to very small size.

The order is: F < Cl > Br > I.

But still the electron affinity of F is greater than those of Br and I.

Conclusion:

We can conclude that the correct option is "2" by comparing the electron affinity values.

Related questions

2) The formation of the oxide ion O^2-_(g) requires first an exothermic and then an endothermic step - as shown below: 

        O_(g) + e^- ====> O^-_(g) ;    ΔH⁰= -142 kJ mol^-1 

        O^-_(g) + e^- ====> O^2-_(g) ;    ΔH⁰= +844 kJ mol^-1  

This is because:

(AIEEE - 2004) 

a) oxygen is more electronegative 

b) oxygen has high electron affinity 

c) O^- ion will tend to resist the addition of another electron 

d) O^- ion has comparatively larger size than oxygen atom

Solution:

When an electron is added to negatively charged ion, it experiences more repulsion rather than attraction. Hence addition of second electron usually requires energy. As a result, second electron affinity values are positive i.e. endothermic.

Conclusion:

The correct option is: 'c'.

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