Group 6 and group 7 elements

[csx]

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why does their reactivity go down as you go down the columns? I would think the Zeff is decreasing and since there are more shells and more shielding, the valence electrons would be more willing to react.

still trying to wrap my head around these periodic trends uggh

 

[Teleologist]

why does their reactivity go down as you go down the columns? I would think the Zeff is decreasing and since there are more shells and more shielding, the valence electrons would be more willing to react.

Wait, so if the charge density of the elements is being diluted by additional shells of electrons, these elements become more reactive?

All other things being equal, something with a +3 charge is more reactive than something with 0 charge. This goes back to Coulomb's law.

We need valence electrons to react, yes, but the number of valence electrons doesn't matter. Charge - REAL (EFFECTIVE) CHARGE* - is the fundamental driving force in chemistry. Nucleophile attacks electrophile. No exceptions.

Chemistry, as I tell other people, is just physics II (aka physics E/M). Just on a more microscopic level.

*Hence I write HO(-) and H3(+)O because these indicate where most of the negative and positive charge, respectively, reside on the molecules. None of that nonsense with writing OH(-) or H3O(+) as if hydrogen were stabilizing a negative charge or oxygen were stabilizing a positive charge. Writing OH(-) tells me you don't know the first thing about electronegativities.

 

[premed1001]

i dont think z eff changes moving up or down. only left or right.
http://forums.studentdoctor.net/threads/effective-nuclear-charge.1073536/
i think the more electronegative an atom is the more it reacts so that may explain your question.
this is a guess i am not positive

 

[Teleologist]

Z eff definitely changes moving up or down due to electron-shell shielding. Z eff is effective nuclear charge. The effective nuclear charge definitely varies with not only the number of protons in the nucleus but also the number of electrons and number of electron shells. The number of protons and electrons all change as we move around on the periodic table. Thus, Z eff changes across the periodic table whether we go up, down, sideways, and/or diagonally.

 

[Czarcasm]

why does their reactivity go down as you go down the columns? I would think the Zeff is decreasing and since there are more shells and more shielding, the valence electrons would be more willing to react.

still trying to wrap my head around these periodic trends uggh

You seem to have that backwards. Reactivity increases as you go down the periodic table. This is why you never encountered Cesium or Francium in lab. They are explosively reactive, especially in the presence of moisture. As for why, well, as we go down a periodic table, each atom is increasing in size (more shell levels). The reactivity has to do with the instability of the valence electrons. Being so far away from where it wants to be (the nucleus), it experiences less attraction and therefore is easily yanked away. Francium is the largest atom in the periodic table, so it's valence electron is furthest away than say, something directly down the same period (where Zeff pulls some of that electron density inwards). So depending on what you're comparing (atoms within the same period or same column), two factors play an important role in reactivity.

 

[premed1001]

Z eff definitely changes moving up or down due to electron-shell shielding. Z eff is effective nuclear charge. The effective nuclear charge definitely varies with not only the number of protons in the nucleus but also the number of electrons and number of electron shells. The number of protons and electrons all change as we move around on the periodic table. Thus, Z eff changes across the periodic table whether we go up, down, sideways, and/or diagonally.

Z effective is total electrons minus all the electrons of lower shells, so how would it change moving up or down the table?

 

[Czarcasm]

Z eff definitely changes moving up or down due to electron-shell shielding. Z eff is effective nuclear charge. The effective nuclear charge definitely varies with not only the number of protons in the nucleus but also the number of electrons and number of electron shells. The number of protons and electrons all change as we move around on the periodic table. Thus, Z eff changes across the periodic table whether we go up, down, sideways, and/or diagonally.

Zeff does not change down a column. Compare sodium and potassium, both in the first column. Sodium has +11 protons but 10 core electrons, a Zeff of +1. Potassium has +19 protons, but 18 core electrons, a Zeff of +1. The Zeff is consistent for each column, but increases as you go down a period because the number of core electrons within a period remains constant, while the number of protons increases.

 

[Teleologist]

Zeff does not change down a column. Compare sodium and potassium, both in the first column. Sodium has +11 protons but 10 core electrons, a Zeff of +1. Potassium has +19 protons, but 18 core electrons, a Zeff of +1. The Zeff is consistent for each column, but increases as you go down a period because the number of core electrons within a period remains constant, while the number of protons increases.

http://withfriendship.com/images/h/39918/Effective_nuclear_charge-image.png

Your formula is too simplistic. Qualitatively, electrons in the same shell shield other electrons less than electrons in a lower energy shell. We can't just say "electrons" and take them all to be equivalent. Sodium has the greater Z eff versus K due to fewer shells of electrons. Same with Li; Li has the greater Z eff vs. Na because Li has 1 fewer shell of electrons.

 

[premed1001]

http://withfriendship.com/images/h/39918/Effective_nuclear_charge-image.png

.

i saw this too, but i think this is beyond scope of the mcat, not entirely sure tho

 

[Teleologist]

i saw this too, but i think this is beyond scope of the mcat, not entirely sure tho

Well I'm at least 2 years out from taking the MCAT so I'll let someone else decide.

I don't see why it wouldn't be tested seeing that it was even mentioned in the crappy Silberberg gen chem text.

 

[premed1001]

Well I'm at least 2 years out from taking the MCAT so I'll let someone else decide.

I don't see why it wouldn't be tested seeing that it was even mentioned in the crappy Silberberg gen chem text.

most undergraduate websites i search dont mention this concept

 

[Czarcasm]

http://withfriendship.com/images/h/39918/Effective_nuclear_charge-image.png

Your formula is too simplistic. Qualitatively, electrons in the same shell shield other electrons less than electrons in a lower energy shell. We can't just say "electrons" and take them all to be equivalent. Sodium has the greater Z eff versus K due to fewer shells of electrons. Same with Li; Li has the greater Z eff vs. Na because Li has 1 fewer shell of electrons.

That table is referring to the net Zeff per electron. The increasing Zeff charge I was referring to was for a given valence shell. Those are entirely different comparisons, especially when you account for the repulsion due to each individual electron vs. the net + charge experienced by the valence shell. The math gets more complicated. For the MCAT, you only need to concern yourself with the later as that's the primary reason why atomic size decreases within a period.

 

[Czarcasm]

most undergraduate websites i search dont mention this concept

Don't stress over it. Focus on the simple concepts. Anything beyond that would be explained in a passage.

 

[Teleologist]

The math gets more complicated but qualitatively it's easy.

Also I don't see the point of delineating the two "kinds" of Z eff. Size decreases across a period because of greater Z eff for both electrons in a shell and the shell as a whole.

If I pull on a tug-of-war rope, then I'm exerting a force both on you on the other side and your team as a whole.

 

[Czarcasm]

The math gets more complicated but qualitatively it's easy. Also I don't see the point of delineating the two "kinds" of Z eff. Size decreases across a period because of greater Z eff for both electrons in a shell and the shell as a whole.

The MCAT is looking to test people on simple concepts. The problem with the scenario you gave is that you have to consider a number of factors: repulsion due to each electron, orbital size, net positive charge, etc. It complicates what could otherwise be a simple question asking you why atomic size decreases within a period. Why cloud your thinking with that line of complicated thinking?

 

[Teleologist]

The MCAT is looking to test people on simple concepts. The problem with the scenario you gave is that you have to consider a number of factors: repulsion due to each electron, orbital size, net positive charge, etc. It complicates what could otherwise be a simple question asking you why atomic size decreases within a period. Why cloud your thinking with that line of complicated thinking?

So 45T's remain unicorns?