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Author Topic: is lattice energy a major part of the "octet rule"?  (Read 2221 times)

Offline chiralSPO

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Inspired by the discussion started by taregg on the most reactive element (http://www.thenakedscientists.com/forum/index.php?topic=49202.0), I found myself wondering a little more about the ionization states of metals.

Take magnesium, for example. In the gas phase, the first 3 ionization energies are 736, 1445 and 7730 (kJ/mol). From these is it obvious that Mg3+ would be very difficult to prepare, and any compound containing it would react very quickly. However, looking at ionization energies alone, it is not clear why Mg+ would be unstable.

Imagine a sample of gaseous Mg. One could irradiate it with light energetic enough to produce Mg+ selectively, without knocking out that second electron. The disproportionation reaction (2Mg+ → Mg + Mg2+) is energetically disfavorable (ΔΕ = 709 kJ/mol) while the revers reaction is favorable.

This disagrees with all my experience with chemistry in condensed states (solid, liquid and solution). Mg and Mg2+ do not comproportionate.

My proposed solution to this is that the ionic radius of Mg2+ is much smaller than that of Mg+, which allows the ion to get more closely associated with negative ions (or the negative end of polar molecules). This would not effect the gas phase case, as distance between species is much, much greater than their radii, but would allow for greater stabilization of the dicationic species in the condensed state.

Any thought on this?


 

Offline evan_au

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Re: is lattice energy a major part of the "octet rule"?
« Reply #1 on: 15/05/2014 12:55:41 »
Pardon my ignorance of chemistry -feel free to correct my misunderstandings...
  • When we are talking about gas-phase Magnesium, we are talking about electrically-neutral atoms?
  • If we now inject some energy, such as a high-voltage AC electrical discharge, we will temporarily rip some electrons away from their atom, causing them to move in opposite directions - but because the electrons are much lighter, they will move much faster and further before the voltage reverses.
  • If  two Mg+ ions collide (ΔΕ = 736kJ/Molx2 = total 1472), won't this tend to temporarily produce Mg + Mg2+ (ΔΕ = 0+1445 = total 1445 kJ/Mol), with a difference of 27kJ/Mol? (I don't understand "disproportionation " )
  • This temporary state would be a plasma, better described by plasma physics, rather than traditional chemistry?
  • As soon as the discharge stops, the electrons will recombine with the nearest positive ion, again producing neutral atoms.
  • I can see that the ionic radius is important in an ionic substance like MgCl2 or aqueus solution like Mg2+Cl-2. But surely this is not so important in a plasma of neutral Mg?
 

Offline chiralSPO

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Re: is lattice energy a major part of the "octet rule"?
« Reply #2 on: 15/05/2014 14:17:18 »
Ignorance forgiven--this is place of learning after all...  :)
Yes, I mean neutral magnesium atoms in a gaseous state.
Sure, we can remove the electrons with a voltage, with light or some other means...
If we start with 2 Mg+ ions, removing an electron from one costs 1445 kJ/mol. Dumping the electron on another Mg+ only returns 736, leaving a net energy cost of 709. "Disproportionation" is the chemist's word for one substance reacting with itself to form two different products (A + A → B + C). "Comproportionation" is the opposite scheme (A + B → C + C).
"I can see that the ionic radius is important in an ionic substance like MgCl2 or aqueus solution like Mg2+Cl-2. But surely this is not so important in a plasma of neutral Mg?" This is essentially the reasoning that I have for why gas (plasma)-phase thermodynamics would contradict my condensed-phase intuition. I think it is correct, but I was hoping for confirmation or a better explanation from someone else on the forum...
 

Offline evan_au

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Re: is lattice energy a major part of the "octet rule"?
« Reply #3 on: 15/05/2014 21:49:30 »
OK, I see one mistake: I read "736, 1445 and 7730" as the energy to remove 1, 2 or 3 electrons from a magnesium atom, respectively.
In fact, these are the energies to remove 1 electron, 1 more electron and 1 more electron...
 

Offline chiralSPO

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Re: is lattice energy a major part of the "octet rule"?
« Reply #4 on: 21/05/2014 21:38:06 »
OK, I see one mistake: I read "736, 1445 and 7730" as the energy to remove 1, 2 or 3 electrons from a magnesium atom, respectively.
In fact, these are the energies to remove 1 electron, 1 more electron and 1 more electron...

correct.
 

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Re: is lattice energy a major part of the "octet rule"?
« Reply #4 on: 21/05/2014 21:38:06 »

 

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