Why do ionic compounds form in the exact ratios that they do instead of more variable ratios? [closed]








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I read that e.g. $\ce{Mg^{2+}}$ and $\ce{Cl-}$ come together because of electrostatic force or coulomb force and form an ionic bond. Then why is the formula $\ce{MgCl2}$? Why does one $\ce{Mg^{2+}}$ attract only two chlorides (if Coulomb's law is at work) and form an ionic bond with two chlorides only? When the ions come close and form a bond, they don't distribute the charge i.e. their electrons in such a way that they would have the same potential. They remain ions even after forming the bond. Then why don't multiple $\ce{Mg^{2+}}$ attract multiple chlorides and form something like $\ce{Mg\_xCl\_y}$? Why does a single molecule of the compound contain one $\ce{Mg^{2+}}$ and two chlorides only?Someone said that,because more clorides would then repel one another so more clorides won't be able to stay with one magnesium ion in one matrix .if we use the coulomb's law kq1q2÷r^2,we should find that the attraction between mg2+ and cl- would be greater than the repulsion between 2 cl-.I mean if we take q1=-1,q2=2,the force will be greater than if we take q1=q2=-1. Then the attraction force between mg ion and cl ion should be able to surpass the repulsion between cl ions and one mg should be surrounded by more than 2 clorides in a crystal.




In some sense, OP is not wrong: $\ce{Mg^{2+}}$ will attract and interact with more than two $\ce{Cl-}$ (and vice versa), in that it will form an "infinite" lattice structure - an ionic crystal (and not molecules, except at extreme conditions that I will ignore here). If you consult any depiction of the structure, you see that every $\ce{Mg^{2+}}$ inside the structure is surrounded (we call it coordinated) by six closest neighbours that are $\ce{Cl-}$.


However, if one were to put only ever more $\ce{Cl-}$ ions around a central $\ce{Mg^{2+}}$, the ions with the same charge would begin repelling each other. To avoid this, $\ce{Mg^{2+}}$ begin grouping themselves around the charge that attracts them. In this way, the crystal forms layer by layer.


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