Question
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Which of the following elements does not follow the phenomenon of inert - pair effect?
A. Sn
B. Pb
C. Tl
D. Bi

Answer Verified Verified
Hint: Inert pair effect is the effect of the s-orbitals in which they do not participate to make bonds with other molecules because the electrons are held tightly due to the strong attraction of the nucleus towards the electrons.

Complete step by step answer:
- We have to identify the elements which cannot show an inert pair effect.
- Firstly, we have to write the electronic configuration of all the elements.
- Now, we know that the atomic number of Sn or tin is 50, atomic no. of lead is 82, Thallium has an atomic number of 81 and an atomic number of bismuth is 83.
- So, the electronic configuration of the elements are:
$\text{Sn (50) = (Kr) 4}{{\text{d}}^{10}}\text{ 5}{{\text{s}}^{2}}\text{ 5}{{\text{p}}^{2}}\text{ }$
$\text{Pb (82) = (Xe) 4}{{\text{f}}^{14}}\text{ 5}{{\text{d}}^{10}}\text{ 6}{{\text{s}}^{2}}\text{ 6}{{\text{p}}^{2}}\text{ }$
$\text{Tl (81) = (Xe) 4}{{\text{f}}^{14}}\text{ 5}{{\text{d}}^{10}}\text{ 6}{{\text{s}}^{2}}\text{ 6}{{\text{p}}^{1}}\text{ }$
$\text{Bi (83) = (Xe) 4}{{\text{f}}^{14}}\text{ 5}{{\text{d}}^{10}}\text{ 6}{{\text{s}}^{2}}\text{ 6}{{\text{p}}^{3}}\text{ }$
- Now, we can see that tin can easily lose two electrons from p-orbital to form +2 oxidation state and also it can lose two electrons from s orbital easily to form +4 oxidation state.
- In tin, the higher oxidation is more stable than the lower oxidation state that’s why it doesn't show an inert pair effect.
- Whereas lead shows the lower oxidation state is most stable that is +2 oxidation state is more stable than the +4 oxidation because the s-orbital is done not releases the electrons.
- Therefore, the lead shows the inert pair effect.
 - Also, thallium shows the lower oxidation state most stable, that is +1 oxidation state is more stable than the +3 oxidation because the s-orbital does not release the electrons.
- Therefore, thallium shows the inert pair effect.
- And bismuth also shows the lower oxidation state is most stable, that is +3 oxidation state is more stable than the +5 oxidation because the s-orbital is done not releases the electrons.
- Therefore, bismuth shows the inert pair effect.
Therefore, option A is the correct answer.

Note: The reason for the inert pair effect is that the elements having the f-orbital show poor shielding effect due to their diffused shapes. As a result the nuclear charge on their s orbitals increases due to this poor shielding effect. This results in more attraction and binding of the electrons in the s-orbitals which stops them from getting out and prevents the metal to acquire a +4 oxidation state.