Having a problem with atom/ion radius

[walker]

Okay I think this question has been addressed slightly before however I seem to be having difficulty on it still.

The question asks: Arrange the following atoms or ions in order of increasing radius Cl, S2-, K, K+, O

Now I have them arranged as

O < Cl < K+ < S2- < K

The arrangement of K+ and S2- is where I am stumped. The question goes on to ask "Give an explanation for the position of S2- in relation to the atom or ion that comes just before and just after" Now I would only assume they're talking about the K+ and K atom/ion so I placed S2- in between. Now my best guess at why S2- is where it is, is due to the gain in electrons. But how do I prove this? Is it because the S atom has gained two electrons and the K atom has only lost one that the S2- ion becomes larger than the K+ ion?

Is there a certain method I should be using to determine the actual resultant size of these atoms when they gain or lose electrons?

I tried to keep the post as informative as possible I'm not trying to scam any answer out of anyone I'm trying to learn how to solve this problem.

 

[chem_tr]

Hello, http://www.scescape.net/~woods/elements/sulfur.html mentions about sulfur's neutral and anionic radii. Potassium atom has the radius of 2.27 angströms, and 1+ cation has 1.52 angströms (according to the same website, change the final part as potassium.htm). Since S^2- is 1.70 angströms, it is much larger than potassium cation. The best explanation would be listing their respective radii with them, the sorting will automatically be done.

 

[walker]

Hello, http://www.scescape.net/~woods/elements/sulfur.html mentions about sulfur's neutral and anionic radii. Potassium atom has the radius of 2.27 angströms, and 1+ cation has 1.52 angströms (according to the same website, change the final part as potassium.htm). Since S^2- is 1.70 angströms, it is much larger than potassium cation. The best explanation would be listing their respective radii with them, the sorting will automatically be done.

Only problem is I believe that stating the exact atomic radius is beyond the scope of the question and this part (if not all) of the course. I also believe that just simply stating their atomic radius does not explain fully why S2- is located where it is.

 

[Gokul43201]

First let's prove that : K+ < S2-

These two are isoelectronic species (they both have 18 electrons). So, the difference between their ionic radii comes down to the difference in their nuclear charge. K+ has 19 protons while S2- only has 16. Due to the larger nuclear charge, K+ must necessarily be smaller (as the attraction towards the nucleus is greater).

Now compare K and S2-

K must be bigger because you are just starting to fill the 4th shell, while S2- only has electrons in 3 shells. However as you go along the period from K to Ca, Sc, etc. the radius decreases making the judgement difficult. This argument is not complete, as it does not compare nuclear charges. While S2- has a smaller nuclear charge thanK, the increase in radius due to this, is not sufficient to overcome the incresing size due to the addition of a new shell. In fact, P3-, which would be bigger than S2- (by the isoelectronic species argument), is still smaller than K.

Warning : Don't not use the Isoelectronic Species comparison to the Noble Gases. They can be weird !

 

[walker]

First let's prove that : K+ < S2-

These two are isoelectronic species (they both have 18 electrons). So, the difference between their ionic radii comes down to the difference in their nuclear charge. K+ has 19 protons while S2- only has 16. Due to the larger nuclear charge, K+ must necessarily be smaller (as the attraction towards the nucleus is greater).

Now compare K and S2-

K must be bigger because you are just starting to fill the 4th shell, while S2- only has electrons in 3 shells. However as you go along the period from K to Ca, Sc, etc. the radius decreases making the judgement difficult. This argument is not complete, as it does not compare nuclear charges. While S2- has a smaller nuclear charge thanK, the increase in radius due to this, is not sufficient to overcome the incresing size due to the addition of a new shell. In fact, P3-, which would be bigger than S2- (by the isoelectronic species argument), is still smaller than K.

Warning : Don't not use the Isoelectronic Species comparison to the Noble Gases. They can be weird !

wow man thanks for the help here... this question was getting to me

one of those questions i guess when you finally get the answer its like "What the hell i should have known that!"