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While studying the topic of chemical bonding, I came across a statement given by my teacher: Elements of the third period and above do not show hybridisation. An example, in the case of $\ce{PH_3}$, $99.9$% s-character exists in the lone pair due to $\ce{P-H}$ bonds being weaker.

Is this the exact statement of Drago's rule? On various other sources, I have found conflicting views, some say that hybridization actually exists and the actual statement is different, the one being taught to students is wrong.

What is Drago's rule? Does it really exist?

I had gone through this answer, it basically explains a quantum mechanical reason for explaining the observation. What I'm seeking is the exact statement of Drago's rule.

For example, in $\ce{PH3}$ does the lone pair exist in pure s-orbital or in a hybrid orbital that is not $\mathrm{sp^3}$ but some other orbital whose nature can be explained and generalized by Drago's rule?

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  • $\begingroup$ Unfortunately, the idea that the lone pair on PH3 is nearly a pure s orbital has long ago been debunked. Linus Pauling popularized the idea in the 1930's, but modern molecular orbital theory showed it to be false. $\endgroup$ – Andrew Jul 24 '20 at 12:35
  • $\begingroup$ @Andrew Can you please shed some more light on that? The pure s-character of the lone pair was told to me by a professor on another forum and he had made this statement with quite a conviction. $\endgroup$ – Display_name Jul 24 '20 at 13:44
  • $\begingroup$ @Aniruddha Deb I had gone through this answer, it basically explains a quantum mechanical reason for explaining the observation. What I'm seeking is the exact statement of Drago's rule. For eg. In PH3 does the lone pair exist in pure s-orbital or in a hybrid orbital that is not sp3 but some other orbital whose nature can be explained and generalized by Drago's rule? $\endgroup$ – Display_name Jul 24 '20 at 13:48
  • $\begingroup$ Drago's rule is :**For any Group in general,the formula for calculating the number of hybrid orbitals is group number of central element + no.of atoms surrounding the central element(except oxygen)-charge on the central element(consider the charge including its sign) whole divided by 2**. I pulled it from here. It seems to be in only Indian Textbooks and is an extension(albeit weak) using the assumptions of Bent's rule $\endgroup$ – Safdar Faisal Jul 24 '20 at 14:26
  • $\begingroup$ A weird coincidence? Search "Drago's rule made by" on google (forgive grammar). $\endgroup$ – Safdar Faisal Jul 24 '20 at 14:27