Effective Nuclear Charge Trend?

[SyrianHero]

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I am so confused right now. I got the new EK books for the 2015 MCAT and the Chem book says on page 9 that the Effective Nuclear Charge increases as you go down a group on a periodic table, but every other resources says otherwise. Can anyone shed some light on this please?

 

[erythrocyte666]

Effective Nuclear Charge should decrease down the group due to shielding from shells beneath. Is that EK's exact wording?

 

[amy_k]

Down a group the number of protons and therefore nuclear charge increases, BUT the number of shielding electrons increases more significantly, which more than counteracts the increasing nuclear charge... resulting in an overall decreasing effective nuclear charge down a group. I am also interested in the exact wording before ruling it a mistake.

 

[nontradalex]

I had this same issue. EK says that Zeff increases from top to bottom because while there is an addition of higher electron shells the increasing positive nucleus outweighs the effect.
I looked up Zeff values and EK is correct.
https://en.wikipedia.org/wiki/Effective_nuclear_charge

Why does every other resource say that Zeff decreases down a group?

 

[BerkReviewTeach]

Why does every other resource say that Zeff decreases down a group?

Probably because every other source focuses on the fact that there are more than two valence electrons once you go into the third row and lower, making Zeff less important. In a nutshell, Zeff increases left-to-right to a significant extent, but going down a column it changes minimally (increasing every so slightly from row 2 to row 3 and row 3 to row 4, before decreasing ever so slightly from row 4 to row 5). Anal as it may be, BR authors and editors make sure everything based on numbers is validated by cross referencing at least five sources (and more if necessary). We take pride in being the ultimate MCAT reference if you want to know what is definitely right. Feel free to trust wiki here if you wish, but if it comes up on the MCAT you should know that when you descend a column, shell radius (cloud size) is more important that Zeff in determining periodic trends. It's pretty well summarized in the BR book if you want to look there.

 

[ElectricNoogie]

I am so confused right now. I got the new EK books for the 2015 MCAT and the Chem book says on page 9 that the Effective Nuclear Charge increases as you go down a group on a periodic table, but every other resources says otherwise. Can anyone shed some light on this please?

Remember, trends are NOT rules. Even the Octet Rule has plenty of exceptions. The AAMC will never expect you to be an atomic property expert. Most books (not chemistry textbooks) focus on the main trend, that as you descend a group, you are adding more and more protons to the nucleus, but the valence electrons are at ever expanding (more distant) energy levels. As a result, the atoms in the lower left corner have much larger radii than the atoms in the upper R corner. Thus, the general trend that would explain this is that Eff Nuc Charge decreases as one goes down a family. Is it exact, nope, but it is in line with the MCATs method of testing these kinds of trends.

The same treatment by the AAMC goes for electronegativity, electron affinity, atomic radius and ionization energy. If you can master the Effec Nuc Charge trend, all the rest make sense.

HERE is a great summary of the trends that is scientifically accurate yet not too dense for MCAT review.

Good luck!

 

[keepSwimmingNemo]

im still confused on effective nuclear charge