[lokifenrir96]

Hi, I read on Wikipedia that a cation with noble gas configuration produces better shielding and less polarising power, hence the bond would have more ionic character and less covalent character (http://en.wikipedia.org/wiki/Fajans'_rules). This is related to Fajan's rules, by the way.

Can someone explain to me the reason why a noble gas configuration produces this effect? Wouldn't losing electrons to achieve a noble gas configuration result in fewer electrons to produce a shielding effect and hence increase polarising power? I can't wrap my head around it... Thanks!

[Dan]

You have to consider a like-for-like comparison, and bear in mind that some factors are more important than others.

If you compare two cations of similar size and charge, e.g. Mg²⁺ and Cu²⁺, Fajan's rules predict that compounds of copper will be more covalent than those of magnesium.

[lokifenrir96]

So you're saying that in general, cations which attain noble gas configuration have a lower core charge than other cations of equivalent size and charge but which do not attain noble gas configuration, and therefore the latters' compounds are more covalent? And that this only applies in such a specific context, so it can't be used to compare, say, two cations with different sizes and charges with one having a noble gas configuration?

Thank you!