[Medicinal]

Hi all.

A. Lithium has an electronegativity value of 1.0, which is higher than Na (0.9) and K (0..

B. Also, Lithiums valence electron is in the 2s orbital. This is much closer to the nucleus compared with the valence electron of Na or K. This means  it is lower energy - more stable.

So how come lithium is unable to hold on to its electron as well as Na or K can? Lithium is at the top of the activity series for metals. Which means it is a stronger reducing agent than both Na and K.

Point A and B given above suggest that the valence electron should be held to the lithium nucleus more than the valence electrons of Na and K. However, this would put Na and K above Lithium on the activity series - and yet they aren't. Can someone clear this up for me?

Also, when calculating electronegativity values for a polyatomic ion, do you average out the electronegativity?

Thanks

[Dan]

So how come lithium is unable to hold on to its electron as well as Na
or K can?

Not true, the ionisation energy of Li>Na>K: Link

Lithium is at the top of the activity series for metals. Which means it is a stronger reducing agent than both Na and K.

Not true: Link

Li is higher than than Na and K in the electrochemical series though, which I think may be what you meant. What is the reaction that the electrode potential is a measure of? Why is it not the same as the ionisation energy?

[Medicinal]

 

This is straight out of my textbook (activity series of the metals):

Li
K
Ba
Ca
Na
Mg
Al
Mn
Zn
Cr
Fe
Cd
Co
Ni
Sn
Pb
H2
Cu
Hg
Ag
Au

i've seen other textbooks with Lithium at the top as well...

I realise that this list isn't complete, but notice how Lithium is above Na and K? As well as everything else.

I can't find it in the textbook at the moment, but in terms of displacement reactions (which lithium is suppose to be very useful for), I understand the following:

Li + NaCl => LiCl + Na

Lithium displaces Na by giving it an electron. However, that would mean Na has a stronger pull on electrons thn Li.

So i'm really confused now.

I must reiterate, this is not the first textbook i've come across that puts Li above Na, and K.

[Arkcon]

Let me give you another Wikipedia link: http://en.wikipedia.org/wiki/Electronegativities_of_the_elements_%28data_page%29  That page cites 3 references for its values, and the literature references they used.  They all give lithium 0.98.  Your textbooks may be wrong.  Not for sure, but a possibility.

[Medicinal]

Let me give you another Wikipedia link: http://en.wikipedia.org/wiki/Electronegativities_of_the_elements_%28data_page%29  That page cites 3 references for its values, and the literature references they used.  They all give lithium 0.98.  Your textbooks may be wrong.  Not for sure, but a possibility.

Thank you for the link. My textbook gives the same electronegativity values (although they are rounded). Lithium 1.0: (rounded), Sodium: 0.9 (rounded) and potassium: 0.8 (rounded).

The electronegativity values aren't in dispute. What was confusing me was why they didn't have the expected affect on the activity series.

So now i'm thinking that the textbook is wrong based on what it says further down in the activity series wikipedia article - SRP are sometimes assumed to be the same as the activity series.

I'm gonna think about SRP and ionisation energies for awhile then post back.

Thanks for you help Arkcon and Dan.

[Dan]

I think the problem is just that the textbooks are referring to activity series when they mean electrochemical series - which is based on electrode potential. Li has a higher gas phase ionisation energy than Na or K as we'd expect from electonegativity, but there are other factors - consider:

What is the reaction that the electrode potential is a measure of? Why is it not the same as the ionisation energy?

[PIQgoogleme]

I've wondered about this too, and here's what I figured out:

Consider the formation of lithium fluoride and sodium fluoride to see why Li is higher on the activity series.

OK if we were making these compounds what would we have to do? First we'd make the ions.

Na Na⁺ + e^-
Li   Li⁺ + e^-

Okay, that took more energy for lithium. Sodium has the lead right now. But consider the next step.

Na⁺ + F^-   NaF
Li⁺ + F^-   LiF

Li has a smaller radius, and they both have the same charge, so by the potential energy equation which you might remember from physics (kq₁q₂/r), this is going to be a lot more exothermic for Lithium, so Lithium wins in the end.

I was never taught this, it's just what I figured when I learned about the activity series. Correct me if I'm wrong.

[Borek]

Li has a smaller radius, and they both have the same charge, so by the potential energy equation which you might remember from physics (kq₁q₂/r), this is going to be a lot more exothermic for Lithium, so Lithium wins in the end.

I was never taught this, it's just what I figured when I learned about the activity series. Correct me if I'm wrong.

You are on the right track. Ionization energy is only part of the picture. In water solutions ions are solvated, the smaller (and more charged) the ion the stronger the attraction of water dipoles.

[Dan]

Yeah, this is what I was getting at. The electrode potential relates to the formation of aqueous ions, the energy change of which can be viewed as a combination of gas phase ionisation and hydration.