Having a problem with atom/ion radius

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In summary, the question asks which atom or ion has the smallest radius, and sulfur (S2-) has the smallest radius of all the elements. The radius of sulfur is 1.70 angstroms, which is much larger than the radius of potassium (K+). The best explanation is that sulfur has gained two electrons, which makes the S2- ion larger than the K+ ion.
  • #1
walker
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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.
 
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  • #2
Hello, http://www.scescape.net/~woods/elements/sulfur.html [Broken] 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 S2- 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.
 
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  • #3
chem_tr said:
Hello, http://www.scescape.net/~woods/elements/sulfur.html [Broken] 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 S2- 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.
 
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  • #4
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 ! :yuck:
 
  • #5
Gokul43201 said:
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 ! :yuck:

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!"
 

1. What is the difference between atom and ion radius?

The atom radius is the distance from the center of the nucleus to the outermost electron shell, while the ion radius is the distance from the center of the nucleus to the outermost electron in an ion. Ions have a different number of electrons than their neutral atom, resulting in a difference in their radii.

2. How does the size of an atom/ion affect its chemical properties?

The size of an atom/ion can affect its chemical properties in several ways. A larger atom/ion will have a greater electron cloud, making it easier to lose or gain electrons and thus more reactive. Smaller atoms/ions have a stronger attraction between the nucleus and electrons, making them less reactive.

3. What factors can influence the atom/ion radius?

The main factors that can influence the atom/ion radius are the number of protons and electrons, the electron configuration, and the presence of additional electrons or a positive charge in the case of ions. The type of element and its placement on the periodic table can also play a role in determining its radius.

4. Can the atom/ion radius be measured directly?

No, the atom/ion radius cannot be measured directly because the electrons are constantly moving and do not have a clearly defined boundary. However, scientists can estimate the radius through various techniques, such as X-ray crystallography or theoretical calculations based on the electron configuration.

5. How does the atom/ion radius change across the periodic table?

The atom/ion radius generally decreases from left to right across a period on the periodic table due to an increase in the number of protons, resulting in a stronger attraction to the electrons. It also increases from top to bottom within a group due to the addition of electron shells, resulting in a larger electron cloud and a larger radius.

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