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My textbook simply tells that ,"since,an ionic compound consists of equal number if positive and negative ions,so the overall charge on an ionic compound is zero." But,what is the reason for the number of positive and negative ions to be equal? Is it not possible that an ionic compound can have unequal number of negative and positive ions?why?

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Sodium needs to lose 1 Electron to attain stable electronic configuration and chlorine needs to gain 1 electron to stable electronic configuration.

In a big picture, the electron was transferred from sodium to chlorine in the same neutral crystal. No electron was supplied to the crystal from outside, it was already in the same system before and after the formation of NaCl.

If the system before formation of NaCl was neutral then it will be neutral even after the formation of NaCl crystal. That may be the reason.

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Well, if the no. Of positive and negative ions were unequal, then the ionic species would have a net charge, i.e., there would be charge separation. Now since electrostatic forces (the forces that bind the cations and anions in an ionic compound) are very strong over short distances (as found in a compound), very large amount of work has to be done to separate the charges. Thus, energy required will be very high.

However, normally a system tends to be in the lowest energy state. So, unless you provide huge amounts of heat or some other external work to separate the charges, the ionic compound will have equal number of cations and anions

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