Place the ions in order of decreasing size. Te2-, I-, Cs+, Ba2+, La3+?

Question

I would like to clarify the order of ion sizes for the following ions: Te²⁻, I⁻, Cs⁺, Ba²⁺, and La³⁺. I initially proposed the following order from largest to smallest: Te²⁻ > I⁻ > Cs⁺ > Ba²⁺ > La³⁺. However, I later confirmed that this answer is incorrect. The alternative order I entered was Cs⁺ > Ba²⁺ > La³⁺ > Te²⁻ > I⁻. Could someone please help me determine the correct order of these ions in terms of decreasing size? Thank you!

Chelsey Frami DVM · Accepted Answer

I believe the answer would be:Te2- > I- > Cs+ > Ba2+ > La3+ (from biggest to smallest)This is what you wrote above.  This makes sense because all 5 ions are isoelectronic with xenon (i.e. they all have 54 electrons) but the number of protons (positively charged particles in the nucleus of the atom) of each is different:La3+ has 54 electrons and 57 protons, meaning that there are 3 additional positive charges to attract the negatively-charged electrons, bringing them closer in towards the nucleus (a "stronger" grip, and therefore, smaller-sized ion if you can imagine).  Ba2+ has 54 electrons but 56 electrons (only 2 additional positive charges).  Cs+ has 54 electrons and only 55 protons (only 1 additional positive charge).I- has 54 electrons and only 53 protons (1 fewer positive charge).Te2- has 54 electrons and only 52 protons (2 fewer positive charges).Anions (negatively charged ions) that are isoelectronic with a particular noble gas (in this case, xenon) are generally larger in size than cations (positively charged ions) that are also isoelectronic with the same noble gas.

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Place the ions in order of decreasing size. Te2-, I-, Cs+, Ba2+, La3+?

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