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While reading about p-block I got to know that in Group 15 elements

electronegativity value decrease down the group but amongst the heavier elements difference is not that much pronounced.

I understood that electronegativity decrease down the group as the atomic size increase but I don't understand why in heavier elements electronegativity difference is not significant. What is the reason for this?

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electronegativity value decrease down the group but amongst the heavier elements difference is not that much pronounced.

The reason for this is that in heavier elements much more filling of inner d and f orbitals starts. For example as we move down from Sb to Bi, just 6 (3+3) more electrons fill p sub-shell, and 2 electrons enter to s sub-shell. Whereas on the other hand, 24 (10+14) electrons make their way to d and f sub-shell. This results in much poor shielding of electrons in outermost shell and effective nuclear charge thus remains approximately same in heavier elements. So, they have nearly same value of electronegativity on electronegativity scales.

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    $\begingroup$ Thanks for the answer. I was also thinking something like this. $\endgroup$
    – Dewansh Chaturvedi
    Commented Dec 17, 2019 at 1:10
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    $\begingroup$ For period 6 there is an additional effect: because the 1s electrons are moving so quickly, they experience relativistic effects, causing the atomic orbitals to become contracted and lower in energy. This increases period 6 elements' electronegativity. $\endgroup$
    – atbm
    Commented Jan 15, 2020 at 15:39

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