[govibe]

can anyone explain to me why the order of these ions in decreasing size is

La3+ > Eu3+ > Gd3+ > Yb3+

[Emily]

Sure... this is pretty easy once you know the tricks of the periodic table. Elements (and ions, for that matter) decrease in size as you move from LEFT to RIGHT across the periodic table. Therefore, because La³⁺ is #57, it's to the left of Eu³⁺, etc.

Hope this helps! Let me know if you need further explanation...

[Borek]

Elements (and ions, for that matter) decrease in size as you move from LEFT to RIGHT across the periodic table.

So why Cl^- is much larger than Na⁺? 

[cliverlong]

Elements (and ions, for that matter) decrease in size as you move from LEFT to RIGHT across the periodic table.

So why Cl^- is much larger than Na⁺? 

Isn't that because when Na is ionised to Na⁺ one has removed the last electron in the third "shell" or orbit and the electrons in the remaining inner "shell" (number 2) are much closer to the nucleus.

I'm being a bit hand-waving because I suspect trying to be rigid about the paths or orbits or locations of electrons in atoms (or indeed anywhere) is a problem due to quantum effects)

Clive

[Borek]

In one ion there is one electron more, in the second - one electron less than in neutral atoms, so they can't be directly compared.

What I was pointing at is that blind use of rules like the one listed is dangerous - you have to understand what and when can be compared.