Ionic structures and bonding
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This question is from the Study Task on page 66 of the Nuffield students' book. What difficulty is involve in measuring the radius of a sodium chloride ion?
Refer to the electron density map for sodium chloride on p 67. You could easily and quite accurately measure the distance from the centre of a sodium ion to the centre of a chloride ion. This distance, you could say, is the sum of the two radii. But is this really true? Do you notice that the two ions distort each other where they get close to each other – the contours are not circular/spherical as they are when closer to the nuclei. Where does one ion end and the other begin? Would it be fair to measure the distance between two chloride ions and halve this measurement to get a radius for a chloride ion? Can you be sure that two adjacent chloride ions are actually touching each other? What does ‘touching’ mean, anyway, if electron clouds are diffuse and not precisely defined?
Ulex
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updated: 22 August 2003
