Why do isosteric molecules have similar shapes? [closed]








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**Isosteres** (*Definition*): `Isosteres are atoms, molecules, or ions of similar size containing the same number of atoms and valence electrons.`[1]


***Questions:***


A. Why should they have same molecular structure (or geometry)?[2][3]


B. Are there exceptions where isosteres don't have same structure? If yes, please give one.


1. [Chemcool Dictionary, Definition of Isosteres](https://www.chemicool.com/definition/isosteres.html#:~:text=Isosteres%20are%20atoms%2C%20molecules%2C%20or%20ions%20of%20similar%20size%20containing%20the%20same%20number%20of%20atoms%20and%20valence%20electrons.)   
 2. [Wikipedia: Isostere](https://en.wikipedia.org/wiki/Isostere#:~:text=or%20ions%20with-,similar%20shape,-and%20often%20electronic)   
 3. [Chemcool Dictionary, Definition of Isosteres](https://www.chemicool.com/definition/isosteres.html#:~:text=In%20general%2C%20isosteric%20molecules%20have%20the%20same%20shape.)




OK, let's get this straight. "**Iso**" means "***same***" and "**stereos**" means "***shape***", so "**isosteric**" means "***having the same shape***". That's what it means because that's what it says, as clearly as the day. There is nothing more to it.


This is the correct definition, and the other one is wrong. See, butane and isobutane contain the same number of atoms (even atoms *of the same elements*) and of valence electrons, so what? Do they have the same shape? Definitely no.


So it goes.


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