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### Topic: Which has the larger atomic radius, S2- or K+ ?  (Read 24243 times)

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#### happydude

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##### Which has the larger atomic radius, S2- or K+ ?
« on: April 30, 2006, 09:06:45 AM »
Hi,

Which has the larger atomic radius, S2- or K+ ?  Both have the same electron configuration 1S2 2S2 2P6 3S2 3P6 but I'm thinking that since K+ has more protons, it would attract the electrons more, causing the electron cloud to be smaller - hence a smaller radius than S2-.  Is this correct?
Thanks

Earl
« Last Edit: May 20, 2006, 12:50:49 PM by Mitch »

#### xiankai

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« Reply #1 on: April 30, 2006, 11:53:41 AM »
sounds alrite to me
one learns best by teaching

#### edimerz

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« Reply #2 on: May 19, 2006, 12:12:54 AM »
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!"

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#### mike

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« Reply #3 on: May 19, 2006, 12:23:34 AM »
edimerz: did you read this before you copied it from the physics forum?
There is no science without fancy, and no art without facts.

#### edimerz

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« Reply #4 on: May 19, 2006, 03:36:11 AM »
Yes

#### edimerz

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« Reply #5 on: May 19, 2006, 04:12:17 AM »
I just figured from the rules of posting:

8.) Please do a Search before posting a new topic. There is no need to take double the space on our servers hosting a topic that has already been discussed previously.

Sorry, for posting it again.
Mikey.

#### syko sykes

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