dielectric constant

[chiddler]

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Why does ionization decrease the dielectric constant?

"...which will not distort as much in the external electric field, because there is less of the atom remaining to distort."

?

 

[SaintJude]

Damn, this must be from TBR. Is this passage-based? Sorry not to answer your q...

 

[chiddler]

Damn, this must be from TBR. Is this passage-based? Sorry not to answer your q...

is ok :-3

no it's just a post-reading question.

 

[MedPR]

Can you type out a little more so its easier to put that quote in context?

If you ionize a dielectric, you are increasing its overall charge (either + or -, I don't think it matters). Dielectrics work by reducing the buildup of charge on the capacitor plates. If you bias the dielectric (by making it more positively or more negatively charged) it won't be able to distribute the charges on the capacitor plates as well and the capacitor plates would not be able to store as much charge as they would with an unionized dielectric.

 

[chiddler]

This is not in the context of capacitors. It's just plain electrostatics:

When some of an atom's charge is removed, it is said to be ionized. What is left is a charged ion which will not distort as much in an external electric field, because there is less of the atom remaining to distort. Therefore polarizability of the atom drops upon ionization (eg, for atomic hydrogen, the polarizability is zero after ionization).

 

[milski]

This is not in the context of capacitors. It's just plain electrostatics:

When some of an atom's charge is removed, it is said to be ionized. What is left is a charged ion which will not distort as much in an external electric field, because there is less of the atom remaining to distort. Therefore polarizability of the atom drops upon ionization (eg, for atomic hydrogen, the polarizability is zero after ionization).

The hydrogen example should give you a pretty good idea. With only a single proton and no negative charge to distribute you cannot polarize it at all. It works in a similar way when you lose a single electron on a larger atom - you have less charge to distribute so even moving most of it to one side you don't get such a big effect as you would if you had an extra electron.

 

[chiddler]

The hydrogen example should give you a pretty good idea. With only a single proton and no negative charge to distribute you cannot polarize it at all. It works in a similar way when you lose a single electron on a larger atom - you have less charge to distribute so even moving most of it to one side you don't get such a big effect as you would if you had an extra electron.

I can make sense of this with a net positive charge (or hydrogen which loses all electrons, losing all capacity for polarizability) because, like alkali metals, the atomic radii become smaller upon losing their valence e- and the remaining electrons are strongly bound to nucleus.

But why would this be the case with a negative charged ion? It's a very large electron cloud that it holds and it doesn't hold it as tightly as does the neutral version. So i'd expect it to be more polarizable in an electric field.

 

[milski]

I can make sense of this with a net positive charge (or hydrogen which loses all electrons, losing all capacity for polarizability) because, like alkali metals, the atomic radii become smaller upon losing their valence e- and the remaining electrons are strongly bound to nucleus.

But why would this be the case with a negative charged ion? It's a very large electron cloud that it holds and it doesn't hold it as tightly as does the neutral version. So i'd expect it to be more polarizable in an electric field.

It is not. Negatively charged ions are highly polarizable. The text does not express it very explicitly with 'removing charge from the atom', but you cannot really remove a positive charge.

 

[chiddler]

It is not. Negatively charged ions are highly polarizable. The text does not express it very explicitly with 'removing charge from the atom', but you cannot really remove a positive charge.

It does. I was lazy and didn't type it all out properly, but I understand now 😛

thanks very much.

 

[SaintJude]

Chiddler, you say you undertstand now. Do you mind summarizing the reason?

 

[chiddler]

Chiddler, you say you undertstand now. Do you mind summarizing the reason?

no problem.

An electric field is weakened if there is a media within the electric field that is polarizable. This is possible because the electrons are tugged towards the positive direction (opposite the Efield lines) and the protons are tugged towards the negative (along the Efield lines) and this makes a small and weak Efield. This electric field of the distorted molecule will partially cancel out the external electric field, reducing the net electric field in that region of space.

When it is ionized, specifically when it loses an electron, there is a net positive charge causing the nucleus to pull electrons strongly. This has two effects:

1. There are fewer electrons being tugged to one side and therefore the Efield created is weaker.
2. The strongly pulled electrons cannot move as much due to external Efield which thus weakens the small internal Efield.

 

[SaintJude]

In both cases above you were referring to the electric field due to polarization, not the net electric field right?