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I'm not a chemistry expert and merely a high school student. In my book there is a slight introduction to VSEPR theory and therein they mention that lone pair-lone pair repulsions are maximum, followed by lone pair-bond pair and then bond pair-bond pair repulsions.

Now the reasoning they provide to this is that as lone pairs are localized on the central atom, and each bond pair is shared, the lone pair occupies more space and hence the repulsions are greater.

Now I don't understand how localisation of lone pair leads to its large size and secondly what is meant by"size"here, because it's hard to imagine size of a lone pair for me.

It's a request not to mention about advanced theories like molecular orbital theory, because I'm not familiar with aaj that right now.

EDIT I would say that the other is also in a way similar but I could not find an answer of my question there. That's why I posted it.

Any of your help is appreciated.

Thanks.

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  • $\begingroup$ @Ayushmaan I referred to this but could not get an answer to my question from here. That's why I posted it. If you know the reason , please help. $\endgroup$ – Abhinav Dhawan Jul 23 '17 at 11:09
  • $\begingroup$ At this stage we are thinking in this box, so we view, electrons clouds and bonds as concrete blocks sticking at one place. But they float...move rapidly pushing the surrounding with repulsion and what not...to spread as far apart as possible. Given that lone pair has a clotted electrons cloud and near to nucleus than to bonding pairs, wouldn't it exerts the most repellence asking for biggest space? $\endgroup$ – bonCodigo Jul 23 '17 at 11:26
  • $\begingroup$ This is one of the worst parts of the theory. It would be better not to try and understand it, because any explanation of this effect is only an attempt to make the theory fit the observation. This part has no footing in actual physical phenomena. $\endgroup$ – Martin - マーチン Jul 23 '17 at 14:38
  • $\begingroup$ @bon codigo Sorry, but I still didn't get an answer. I tried to understand yours in this comment section, but it's beyond my understanding. I've edited to explain why I posted this though there is a similar question put there.May be Martin is right , that the answwr is above my level now. Thanks and would be happy to get a correct and simple answer, if possible. $\endgroup$ – Abhinav Dhawan Jul 24 '17 at 0:07
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    $\begingroup$ It is very unfortunate that this theory is taught in such a universal fashion as a truth. (And it has many flaws.) You therefore learn something, that later, when you advance to a more complex understanding, have to unlearn. And that usually is more difficult. The very questions you ask are proof that you would be ready for a deeper understanding and a more fundamental theory. My personal opinion is that molecular geometry can be taught in a different way, not resorting back to oversimplified theories. Stay curious and a bit patient, over time, things will get clearer. $\endgroup$ – Martin - マーチン Jul 24 '17 at 4:10
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The basic premise of this statement is that the repulsion exerted by an electron system depends upon the area it occupies. More the area, more is the repulsion.

As the bond pair is formed from the sharing of electrons, it experiences attraction from not one, but two nucleii. On the other hand, the lone pair is attracted by only one nucleus.

So, the bond pair is more localised while the lone pair is spread out and occupies a larger area.

The 'size' here refers not to the size of electrons but to the size and nature of the orbitals these electrons occupy.

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