Electrostatic charges in ionic compounds

[sgstudent]

From this link: http://www.chemguide.co.uk/atoms/structures/ionicstruct.html

it explains why NaCl has a 6:6 coordinated structure and not an 8:8 coordinated structure. However, i don't really understand their explanation "Now imagine what would happen if you replaced the caesium ion with the smaller sodium ion. Sodium ions are, of course, smaller than caesium ions because they have fewer layers of electrons around them.

You still have to keep the chloride ions in contact with the sodium. The effect of this would be that the whole arrangement would shrink, bringing the chloride ions into contact with each other - and that introduces repulsion.

However, I don't see how the added repulsion would cause it to align in the 6:6 coordination. Won't all the forces be canceled out even in that structure (http://postimage.org/image/hushz6i5z/ ) drawing the forces in the object, i realized that the forces will cancel out so there is not net force on the whole object. So why can't NaCl form an 8:8 coordinated structure?

Thanks for the help :)

 

[Vailanor]

The reason why NaCl cannot form an 8:8 coordinated structure is because the ions are too close together. When the ions are too close together, they experience repulsion due to their electrical charges. This repulsion causes the ions to move further apart, resulting in a 6:6 coordination instead of 8:8. The repulsion of the ions is stronger than the forces that would keep them together in an 8:8 coordination.