Destroyer gchem 203

[txlonghorn]

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Why is S 2- larger than Sr 2+ ? these are not isoelectric, so wouldn't Sr 2+ be larger? The destroyer says that it is false that the strontium cation is larger than the sulfer anion since anions gain electrons.

 

[mashinka]

My understanding is that the more electrons you have, the larger that atom because either there are more orbitals or the nuclear pull by the protons is less strong (because there are more electrons) so the electrons are able to spread out more.
Does that make sense?

 

[FSUpredent]

My understanding is that the more electrons you have, the larger that atom because either there are more orbitals or the nuclear pull by the protons is less strong (because there are more electrons) so the electrons are able to spread out more.
Does that make sense?

Correct. Check out this link and scroll down to the ionic radius section for a concise explanation.

Quote from the link:

IONIC RADIUS

Ions aren't the same size as the atoms they come from. Compare the sizes of sodium and chloride ions with the sizes of sodium and chlorine atoms.



Positive ions

Positive ions are smaller than the atoms they come from. Sodium is 2,8,1; Na+ is 2,8. You've lost a whole layer of electrons, and the remaining 10 electrons are being pulled in by the full force of 11 protons.

Negative ions

Negative ions are bigger than the atoms they come from. Chlorine is 2,8,7; Cl- is 2,8,8. Although the electrons are still all in the 3-level, the extra repulsion produced by the incoming electron causes the atom to expand. There are still only 17 protons, but they are now having to hold 18 electrons.

 

[DoCt0rPeTe]

just remember the general trend

Anions > Neutral > Cations in size and you should be good. The DAT is not gonna expect you to know the weird exceptions.

 

[RSD2014]

Why is S 2- larger than Sr 2+ ? these are not isoelectric, so wouldn't Sr 2+ be larger? The destroyer says that it is false that the strontium cation is larger than the sulfer anion since anions gain electrons.

Radii comparison
S 103pm
S2- 184pm

Sr 215pm
Sr2+ 113pm

It could be that Sr losing 2e- resulted in a positive charge and the Effective nuclear charge pulls the electrons even closer to the nucleus. So the atom gets even smaller.

While S gaining 2e-, electrons repel each other pushing them farther apart, resulting in a larger radius.

 

[UCB05]

It should already be pretty obvious that addition of an electron to create an anion will result in a larger atomic radius because of electron repulsion and relation to nuclear charge, and loss of an electron to create a cation will result in a smaller radius because of the opposite effect. I doubt longhorn's confusion is due to this basic principle.

It so happens that even though Sr2+ is one whole isoelectric period lower than S2-, the effect of repulsion makes the atomic radius of S2- larger and cation attraction makes Sr2+ small enough that Sr2+ is smaller than S2-. This is the same observation as in O2-(126pm) vs Ca2+(114pm). IMO just keep that in mind and don't dwell on it.

 

[DQLEUCD]

Why is S 2- larger than Sr 2+ ? these are not isoelectric, so wouldn't Sr 2+ be larger? The destroyer says that it is false that the strontium cation is larger than the sulfer anion since anions gain electrons.

more electrons means bigger electron cloud. If the electron count isequal, then you look account for nuclear charge where the one w/ more nuclear charge will have a smaller radius.

 

[UCB05]

more electrons means bigger electron cloud. If the electron count isequal, then you look account for nuclear charge where the one w/ more nuclear charge will have a smaller radius.

That's the whole point. The electron counts are not equal. Sr2+ has an entire shell more electrons than S2- They aren't isoelectronic, which turns an easy question into a "huh,wat?" question. So short answer to the original question: Sr2+ is smaller even though it's electronically "larger" by an entire shell. Why? It just is.

 

[txlonghorn]

That's the whole point. The electron counts are not equal. Sr2+ has an entire shell more electrons than S2- They aren't isoelectronic, which turns an easy question into a "huh,wat?" question. So short answer to the original question: Sr2+ is smaller even though it's electronically "larger" by an entire shell. Why? It just is.

I am sure on the DAT they won't ask dumb questions like this one. I guess I will just apply the anion is larger than everyone's mama rule.

 

[chromosome21]

just remember the general trend

Anions > Neutral > Cations in size and you should be good. The DAT is not gonna expect you to know the weird exceptions.

+1

anions are bigger. Having more electrons than protons in the nucleus will make the anion bigger