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Chemistry:

 

I am confused about Zeff and Shielding. I thought as we go from right to left, top to bottom, shielding increases (because of an increase in atomic radius) and Zeff decreases (because the outermost electrons will feel a lower charge/attraction from the protons found in the nucleus).

 

But in a practice question, it said that going from N to O, Zeff decreases which is opposite to the trend I had memorized. Can someone help clarify the concept?

 

Thanks

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It might be that because Nitrogen has a half-filled p-orbital, it gains some extra stability that decreases shielding and increases Zeff. The same might be true of d-block metals such as Chromium that you would expect to have a configuration [Ar]4s^2 3d^4 but actually have [Ar]4s^1 3d^5 because a half-filled d-orbital confers higher stability.

 

Somebody should still correct me if I'm wrong!

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The effective nuclear charge is equal to the number of an atom's protons subtract its non-valence electrons.

 

For nitrogen, this is 7-2 = 5.

For oxygen, it is 8-2 = 6.

 

Oxygen has the higher Zeff so the smaller atomic radius. The amount of shielding is the same in each atom (same # of inner-shell electrons); however, the amount of positive charge that is shielded is different, resulting in different Zeff's.

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Thanks CollegeDude!

 

That is how I was thinking of this question but the solution said that O has a lower Zeff in comparison to N but I don't see how that would be the case.

 

Also, if Zeff = number of protons - number of non-valence electrons, isn't that essentially the same as the number of valence electrons?

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What is the difference between formal charge and oxidation state? I understand that the sum of either of the two if the overall charge on the molecule and oxidation state tells about the relative pull on the electrons.

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Let's look at the Nitrogen atom in nitric acid (HNO3)

 

The formal charge is +1.

 

FC = Valence - Nonbonding - Bonding/2

FC = 5 - 0 - 8/2 = 1

 

Conversely, the oxidation state of the nitrogen atom is +5.

 

Hydrogen has an oxidation state of +1, since it is bonded to a more electronegative element. Oxygen has an oxidation state of -2, since this molecule is not a peroxide. The sum of the oxidation states is equal to the charge.

 

0 = 1 + N + 3(-2)

N = 5

 

This highlights the difference between formal charge and oxidation states. The oxidation state reflects unequal sharing of electrons, whereas the formal charge is just a book-keeping device.

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