Palladium & Nickel

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Palladium & Nickel

#1  Postby LjSpike » May 25, 2016 7:51 pm

Every other element in the transition metals has either 1, or 2 electrons in its outer shell, and on period 5, where Palladium is, all the elements have 5 electron shells, except palladium, which has 4.
Why is palladium the 1 exception, to this beautiful neat pattern?

Then, Nickel, 1 or 2 electrons in its outer shell? How does this work then? Its the only element which does this (as far as I know)!

(Additionally if anyone wishes to answer, why is it a bit 'random', where the electrons go in the transition metals, for example iridium has 2 electrons in its outer shell, bu then platinum and gold have 1 in the outer shell, then mercury has 2 again in the outer shell?)
:?
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Re: Palladium & Nickel

#2  Postby ScholasticSpastic » May 25, 2016 9:19 pm

I think you're coming at the problem backwards. It's the protons that are important. The electrons are merely a means for balancing charge, and what we observe in chemistry seldom reflects what we would expect when considering each atom by itself.

I don't understand where you got the impression that the transition metals almost all have one or two valence electrons. Are you discussing shell filling conventions or the types of ions typically formed? I think some clarification is required here. Don't feel bad. Chemistry was both my favorite course and also the one where I literally cried and threw my book at the wall.

The apparent randomness of which elements are considered to have how many electrons in their outer shell is due to relative energy levels of the various sub-shells. Perhaps this bit from wikipedia will help you out:
Wikipedia wrote:The valence shell is the outermost shell of an atom. It is usually (and misleadingly) said that the electrons in this shell make up its valence electrons, that is, the electrons that determine how the atom behaves in chemical reactions. Just as atoms with complete valence shells (noble gases) are the most chemically non-reactive, those with only one electron in their valence shells (alkali metals) or just missing one electron from having a complete shell (halogens) are the most reactive.[8]

However, this is a simplification of the truth. The electrons that determine how an atom reacts chemically are those that travel farthest from the nucleus, that is, those with the highest energy. For the transition elements, the partially filled (n − 1)d energy level is very close in energy to the ns level[9] and hence the d electrons in transition metals behave as valence electrons although they are not in the so-called valence shell.
https://en.wikipedia.org/wiki/Electron_shell

Things get wonky with the metals because the d-sub-shell electrons are very close, energetically, to the s-sub-shell electrons. Whereas with the non-metals we get to basically ignore all but the s and p sub-shells when considering valence conformations. How comfortable are you with the sub-shells? This is probably the point where you'll find clarity regarding outer shell fullness.
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Re: Palladium & Nickel

#3  Postby zoon » May 25, 2016 9:44 pm

As a complete non-expert, after googling Wikipedia here (redirected from a PhysicsStackExchange question and answer), it looks to me as though the best available rule for predicting which orbitals electrons occupy in the atoms of different elements (the Madelung energy ordering rule) has a number of experimental exceptions, such as copper, chromium and palladium, and nobody yet knows why. The rule seems to have been an experimental rule of thumb in the first place, rather than being derived from theory.
Wikipedia wrote:The rule is based on the total number of nodes in the atomic orbital, n + ℓ, which is related to the energy.[3] In the case of equal n + ℓ values, the orbital with a lower n value is filled first. The fact that most of the ground state configurations of neutral atoms fill orbitals following this n + ℓ, n pattern was obtained experimentally, by reference to the spectroscopic characteristics of the elements.[4]

The Madelung energy ordering rule applies only to neutral atoms in their ground state, and even in that case, there are several elements for which it predicts configurations that differ from those determined experimentally.[5] Copper, chromium, and palladium are common examples of this property. According to the Madelung rule, the 4s orbital (n + ℓ = 4 + 0 = 4) is occupied before the 3d orbital (n + ℓ = 3 + 2 = 5). The rule then predicts the configuration of 29Cu to be 1s22s22p63s2 3p64s23d9, abbreviated [Ar]4s23d9 where [Ar] denotes the configuration of Ar (the preceding noble gas). However the experimental electronic configuration of the copper atom is [Ar]4s13d10. By filling the 3d orbital, copper can be in a lower energy state. Similarly, chromium takes the electronic configuration of [Ar]4s13d5 instead of [Ar]4s23d4. In this case, chromium has a half-full 3d shell. For palladium, the Madelung rule predicts [Kr]5s24d8, but the experimental configuration [Kr]4d10 differs in the placement of two electrons.
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Re: Palladium & Nickel

#4  Postby LjSpike » May 26, 2016 3:13 pm

ScholasticSpastic wrote:I think you're coming at the problem backwards. It's the protons that are important. The electrons are merely a means for balancing charge, and what we observe in chemistry seldom reflects what we would expect when considering each atom by itself.

I don't understand where you got the impression that the transition metals almost all have one or two valence electrons. Are you discussing shell filling conventions or the types of ions typically formed? I think some clarification is required here. Don't feel bad. Chemistry was both my favorite course and also the one where I literally cried and threw my book at the wall.

Things get wonky with the metals because the d-sub-shell electrons are very close, energetically, to the s-sub-shell electrons. Whereas with the non-metals we get to basically ignore all but the s and p sub-shells when considering valence conformations. How comfortable are you with the sub-shells? This is probably the point where you'll find clarity regarding outer shell fullness.

Well, im fine with the concept of the number of protons determining what element an atom is, and that the number of protons must equal the number of electrons. The thing is however, in the period that palladium is, all other elements have their electrons structured, so that it forms 5 (if I remember correctly, I did have a maths exam today, so my mind is still recovering) shells. Palladium however, crams the electrons into its fourth shell, even more than the other transition metals (I've read up on the transition metals being what they are, as for -some- reason they allow more electrons into a shell, putting the maximum at 18 or such).
Then as well, palladium is the ONLY element to have 18 electrons in its outermost (valence) shell.

I've not looked into sub-shells before? Is that how the blocks of the periodic table are divided then, by how the subshells work?
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Re: Palladium & Nickel

#5  Postby ScholasticSpastic » May 26, 2016 3:25 pm

zoon wrote:As a complete non-expert, after googling Wikipedia here (redirected from a PhysicsStackExchange question and answer), it looks to me as though the best available rule for predicting which orbitals electrons occupy in the atoms of different elements (the Madelung energy ordering rule) has a number of experimental exceptions, such as copper, chromium and palladium, and nobody yet knows why. The rule seems to have been an experimental rule of thumb in the first place, rather than being derived from theory.

I think spin-matching may be sufficient to explain most of the exceptional cases. Electrons have two spin states: Spin up and spin down. It is most energetically favorable to have all electrons of both spins in a sub-shell. But the next most energetically favorable state is to have all electrons of one spin state in a sub-shell. The Madelung rule for Palladium is weird. There is no way a 10-member sub-shell would stop at just eight electrons. It is far, far more favorable, energetically, to finish filling the d sub-shell and leave the s sub-shell, which is pretty close energetically, empty.
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Re: Palladium & Nickel

#6  Postby ScholasticSpastic » May 26, 2016 3:27 pm

LjSpike wrote:
I've not looked into sub-shells before? Is that how the blocks of the periodic table are divided then, by how the subshells work?

If you've not gotten to the sub-shells yet, then yeah, it's going to look very confusing. The sub-shells, themselves, take some getting used to, but once you've got a handle on them it'll explain a lot of what you're currently struggling with. Sub-shells are the bit of chemistry where you get to start using the word "quantum" without selling snakeoil. Enjoy that. :beer:
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