Chemical Forums

Chemistry Forums for Students => High School Chemistry Forum => Topic started by: happydude on April 30, 2006, 09:06:45 AM

Title: Which has the larger atomic radius, S2- or K+ ?
Post by: happydude on April 30, 2006, 09:06:45 AM

Hi,

Which has the larger atomic radius, S^2- 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 S^2-.  Is this correct?
Thanks

Earl

Title: Re: Atomic Radius
Post by: xiankai on April 30, 2006, 11:53:41 AM

sounds alrite to me

Title: Re: Atomic Radius
Post by: edimerz 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!"

From the web

Title: Re: Atomic Radius
Post by: mike on May 19, 2006, 12:23:34 AM

edimerz: did you read this before you copied it from the physics forum?

Title: Re: Atomic Radius
Post by: edimerz on May 19, 2006, 03:36:11 AM

Yes

Title: Re: Atomic Radius
Post by: edimerz 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.

Title: Re: Atomic Radius
Post by: syko sykes on May 19, 2006, 12:54:22 PM

make sure what you're posting is actually relevant to the topic you're posting it in edimerz. happydude had essentially already answered his own question and it had already been confirmed as correct so unless you have something that debates that there is no further need to post

sorry for wasting a post ranting but i felt it needed to be done... here i'll try and contribute something too

the electrons in each atom experience the same amount of sheilding so you (happydude) would be correct in thinking that the Sulfur atom would be smaller due to greater attractive forces from the protons