Why is it that Zeff increases going down a group, but atomic radii increases?

lull

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May 29, 2018
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Examkrackers says "Zeff also increases going from top to bottom down the periodic table. Though the energy level of the outermost electrons increases down a group, the attractive pull of the growing positively charged nucleus outweighs the additional shielding effects of higher electron shells". Is there a certain point going down a group that the attractive pull of the nucleus outweighs electron shielding? If this is the case, why would the atomic radii increase if there's more nuclear attractive forces going down a group?

Looking at group 1 for example, it seems that Zeff remains the same at 1 (i.e., the increase in number of shielding electrons is negated by the increase in protons), and consequently atomic radii increases.
 

Screamapillar

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Jun 23, 2013
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Even though the Zeff is greater going down a group, you are still adding more layers of electron shells. The additional strength of the pull of the nucleus in not enough to counteract the fact that the orbitals are spinning so far away from the nucleus. While increased Zeff will pull electrons tighter in the same period, it can't compete with a whole new layer, and the addition of a new shell of valence electrons will always increase radius.
 

lull

2+ Year Member
May 29, 2018
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213
Even though the Zeff is greater going down a group, you are still adding more layers of electron shells. The additional strength of the pull of the nucleus in not enough to counteract the fact that the orbitals are spinning so far away from the nucleus. While increased Zeff will pull electrons tighter in the same period, it can't compete with a whole new layer, and the addition of a new shell of valence electrons will always increase radius.

Doesn't Zeff take into account electron shielding? I agree with you that the increase in electron shells will increase radius, I'm not sure that the Zeff would increase though going down a group if there is more shielding, and Zeff accounts for the shielding.
 

Screamapillar

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Doesn't Zeff take into account electron shielding? I agree with you that the increase in electron shells will increase radius, I'm not sure that the Zeff would increase though going down a group if there is more shielding, and Zeff accounts for the shielding.
Zeff accounts for the shielding, yes, but the Zeff only measures the force the nucleus affects on the valence electrons. There may be other forces acting, such as the forces that sort electrons in farther and farther energy shells. Here's a pretty good explanation I found when trying to answer the question: Atomic radius and nuclear effective charge in a group.

My take away is that, whatever quantum force keeps putting electrons in higher and higher energy levels is stronger than an increase in Zeff. To really understand may require a deeper delve into quantum mechanics than is needed for the MCAT, I would recommend just rationalizing it in a simple way, saying that Zeff increases going down a group, but the principle quantum number forces electrons to occupy farther shells and is stronger than the Zeff increase.
 
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