paramagnetic and diamagnetic

[hermanntrude]

I'm teaching this topic soon and i'm trying to get my knowledge up to scratch. I know that the books usually define a diamagnetic substance as one in which all the electrons are paired. and a paramagnetic substance as one with unpaired electrons.

 

For demonstration purposes I have a small torsion balance set up so I can show the diamagnetism of bismuth, which works beautifully.

 

However, I realised yesterday, that according to everything the textbook says, bismuth ought to be paramagnetic, as it has the electron configuration:

 

[math]Bi: [Xe]6s^24f^{14}5d^{10}6p^3[/math]

 

The three p-electrons ought to be unpaired and hence the element should be paramagnetic.

 

The only explanation I can come up with is that bismuth is BOTH paramagnetic AND diamagnetic at the same time, and so are all elements which have unpaired electrons. Bismuth is a special case because it has so many unpaired electrons (80 in total) that the diamagnetic effect outweighs the paramagnetic effect (which is due to only 3 electrons).

 

I wondered about the relative strength of the two effects per electron

 

In aluminum (which is a well-known paramagnet), there are 12 paired electrons and one unpaired electron. This means that the diamagnetic effect per electron must be less than one-twelfth of the paramagnetic effect per electron.

 

Is this sound reasoning or am i flying off on a tangent? perhaps the effect isnt directly proprtional to the number of electrons? perhaps only para or diamagnetism is expressed at any one time like the textbooks suggest?

[Zolar V]

hmm, i have no idea. but i definitely am excited to see the explanation

[Tartaglia]

Possibly something to do with its band structure and the density of states (My solid state physics isn't great unfortunately)

[John Cuthber]

IIRC the diamagnetism goes away on melting so it's to do with the odd crystal structure,

[DrP]

IIRC the diamagnetism goes away on melting so it's to do with the odd crystal structure,

 

hmm - what about water though.

[hermanntrude]

if you check water's molecular orbital diagram, you'll see that meets the standard definition of diamagnetism

[DrP]

Yea - it's the standard practical demo with the magnet next to the running tap pushing the water stream away. So nothing, iirc, to do with crystal stucture.

[Gilded]

IIRC the diamagnetism goes away on melting so it's to do with the odd crystal structure,

 

You're probably confusing it with the Curie temperature, at which a material's ferromagnetism fades (beyond the Curie point the material is just paramagnetic). I'm not sure if it affects diamagnetism.

 

On diamagnetism, it is affected by the material's structure to some extent. For example diamond is more diamagnetic than normal, randomly oriented graphite, where as pyrolytic graphite is more diamagnetic than diamond.

Edited October 29, 2008 by Gilded

[John Cuthber]

I know what a Curie point is thank you.

Does anyone have access to this paper?

http://www.jstor.org/pss/95646

 

P371 of this

http://www.rsc.org/delivery/_ArticleLinking/DisplayArticleForFree.cfm?doi=CA9058805365&JournalCode=CA

suggests rather strongly that the crystal structure affects the diamagnetism of Bi.

The behaviour of liquid water is, of course, nothing to do with the crystal structure; why mention it?

[big314mp]

Are you asking if we can read it, or are you asking someone to send it to you?

[hermanntrude]

I know what a Curie point is thank you.

Does anyone have access to this paper?

http://www.jstor.org/pss/95646

 

P371 of this

http://www.rsc.org/delivery/_ArticleLinking/DisplayArticleForFree.cfm?doi=CA9058805365&JournalCode=CA

suggests rather strongly that the crystal structure affects the diamagnetism of Bi.

The behaviour of liquid water is, of course, nothing to do with the crystal structure; why mention it?

 

do you know whether my original assessment of the origin of bismuth's diamagnetism was correct, though?

[DrP]

IIRC the diamagnetism goes away on melting so it's to do with the odd crystal structure,........................................................The behaviour of liquid water is, of course, nothing to do with the crystal structure; why mention it?

 

Sorry - forgot you were talking about Bi specifically rather than diamagnetism in general.

 

 

 

I wondered about the relative strength of the two effects per electron

 

In aluminum (which is a well-known paramagnet), there are 12 paired electrons and one unpaired electron. This means that the diamagnetic effect per electron must be less than one-twelfth of the paramagnetic effect per electron.

 

The paramagnetic effect is ment to be stronger - I see your point - I've never actually thought about it. I can't find anything whilst briefly searching around, but the electron thing does seem to make sense. Let us know if you find out!!

[John Cuthber]

I got a reference to that paper from Google and the few fractured sentences that Google gave sugested that the original paper might answer the question of what happens to the diamagnetism of Bi when it melts.

I'd like to know what the whole paper says.

 

There's certainly some effect of crystal structure as shown by the second paper I cited.

[big314mp]

I'd like to know what the whole paper says.

 

My one contribution

[big314mp]

*Note: I need to take this down fairly quickly, since I'm not technically allowed to post it here.

[hermanntrude]

interesting article, but it still doesnt tell me why solid bismuth is diamagnetic when all the textbooks seem to suggest it should be paramagnetic

[John Cuthber]

Big 314MP, thanks for the paper. I think the bit I was looking for is this

"To explain the existence of these cracks Kapitza made the suggestion that in solidifying, bismuth, which normally has a rhombohedral structure, passes through a cubic modification. The bismuth must then change structure while solid, and it would be conceivable that the strains resulting from such a process might be responsible for the development of cracks. This view was based on the experiments of Curie,* who found that the anomalously large dia- magnetism of solid bismuth disappeared at a temperature appreciably below the melting point." The paper goes on to say thet the change in magnetism happens at the melting point rather than near it (as M. Curie thought) but the fact is that molten Bi isn't anomalously diamagnetic so the diamagnetism must arise from the crystal structure.

I think we can assume that M. Curie knew what a Curie temperature was.