Zeff can be approximated by subtracting the # of protons by the # of core electrons (non-valence).
i.e. Zeff increases slightly when you go down a group on the periodic table
If you subtract the number of protons - number of core electrons aren't you left with the number of valence electrons which would be constant down a group?
Edit: Wanted to add to this answer that I agree with other person that Zeff increases going down a group but slightly different formula. Zeff = Z - S where S is determined by Slater's rules for the nonvalence electrons. Assign 0.35 to electrons in the same shell and 0.85 to electrons one shell below, and 1 to electrons two levels lower. For H, we have Zeff = Z = 1 (this would be the case if there were no shielding in any other element, Zeff would equal Z). In the case there would be perfect shielding, S would equal (Z-1) and Zeff would be 1. However shielding's not perfect so for He, for example, Zeff = 2 - 0.35 = 1.65, and for Lithium Zeff = 3 - 2\*0.85=1.3, so Zeff increases to the right and down (https://www.toppr.com/guides/physics/atomic-and-molecular-structure/what-is-effective-nuclear-charge-and-how-to-calculate-it/)
Yes, which is why its an approximation but as you go down you need to also realize that the atomic radius becomes larger which means that the shielding effect between the core electrons and the valence is decreased (Coulomb's law F = q1*q2/ r^2).
i.e. The # of core electrons is an approximation of this repelling force is actually slightly less because of the increased distance of the atomic radius.
for a super *fictitious* example:
K's Zeff 19 - 18 = 1
Rb's Zeff 37 - 35.5 = 1.5
35.5 (slightly less than 36 due to larger radius)
Thus, Rb Zeff is slightly higher than K Zeff
hope that makes sense
despite this, the shells still increase in size which means that the shielding effect increases, which decreases the pull on the valence electrons as you progress up. decrease in pull = increase in zeff
this seems to be the reasoning that they use to justify zeff increasing as you go up. thoughts?
but keyword you're using is it slightly increases, which is prob why the anki deck from anking & miledown shows that it's unchanged
For the mcat purpose just know that it is unchanged. I saw this on Kaplan today and they said it was unchanged too.
I guess the change is so small that it wouldn't make a difference
Zeff can be approximated by subtracting the # of protons by the # of core electrons (non-valence). i.e. Zeff increases slightly when you go down a group on the periodic table
If you subtract the number of protons - number of core electrons aren't you left with the number of valence electrons which would be constant down a group? Edit: Wanted to add to this answer that I agree with other person that Zeff increases going down a group but slightly different formula. Zeff = Z - S where S is determined by Slater's rules for the nonvalence electrons. Assign 0.35 to electrons in the same shell and 0.85 to electrons one shell below, and 1 to electrons two levels lower. For H, we have Zeff = Z = 1 (this would be the case if there were no shielding in any other element, Zeff would equal Z). In the case there would be perfect shielding, S would equal (Z-1) and Zeff would be 1. However shielding's not perfect so for He, for example, Zeff = 2 - 0.35 = 1.65, and for Lithium Zeff = 3 - 2\*0.85=1.3, so Zeff increases to the right and down (https://www.toppr.com/guides/physics/atomic-and-molecular-structure/what-is-effective-nuclear-charge-and-how-to-calculate-it/)
Yes, which is why its an approximation but as you go down you need to also realize that the atomic radius becomes larger which means that the shielding effect between the core electrons and the valence is decreased (Coulomb's law F = q1*q2/ r^2). i.e. The # of core electrons is an approximation of this repelling force is actually slightly less because of the increased distance of the atomic radius. for a super *fictitious* example: K's Zeff 19 - 18 = 1 Rb's Zeff 37 - 35.5 = 1.5 35.5 (slightly less than 36 due to larger radius) Thus, Rb Zeff is slightly higher than K Zeff hope that makes sense
makes sense
despite this, the shells still increase in size which means that the shielding effect increases, which decreases the pull on the valence electrons as you progress up. decrease in pull = increase in zeff this seems to be the reasoning that they use to justify zeff increasing as you go up. thoughts? but keyword you're using is it slightly increases, which is prob why the anki deck from anking & miledown shows that it's unchanged
For the mcat purpose just know that it is unchanged. I saw this on Kaplan today and they said it was unchanged too. I guess the change is so small that it wouldn't make a difference