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Author Topic: What happens when cation anion pairs polarise one another?  (Read 5167 times)

Offline mogsmar5

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Please please please read through all this and correct me if I'm wrong, or if you think I'm misunderstanding something. I want to understand this polarisation business thoroughly!

My understanding of polarisation of compounds so far is:

What you might call 'originally' ionic compounds (i.e, compounds containing metals and non-metals) polarise according to the relative charges of the cation and anion and the relative size of each, some to the extent of becoming covalent. Small ions with high charges have the greatest polarising power, large ions with low charges have the highest polarisability. For an 'originally' ionic compound to be classed as covalently bonded the cation must polarise the anion, and never the other way round.

Where the polarisation of 'originally' ionic compounds is determined by charge, for 'originally' covalent compounds it is determined by electronegativity instead (something I haven't yet covered in class). Rules regarding size are the same as before. Presumably for an 'originally' covalent compound the anion must polarise the cation for it to become ionically bonded, and not the other way round.


Thank you very much,

George


[MOD EDIT - PLEASE ENSURE THAT YOU PHRASE THE TITLES OF YOUR POSTS AS QUESTIONS. THIS IS FORUM POLICY AND YOU ARE ALSO MUCH MORE LIKELY TO RECEIVE AN ANSWER TO YOUR QUERY IF YOU DO SO. THANKS. CHRIS]
« Last Edit: 16/01/2010 10:14:42 by chris »


 

Offline Sparrowhawk

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What happens when cation anion pairs polarise one another?
« Reply #1 on: 17/01/2011 20:30:24 »
The polarising ability of a positive ion depends upon the charge density. This is the charge divided by the radius of the ion.
The smaller the ion, the closer you can get to the centre of the charge. Since the charge is a positive force field spreading out from the nucleus the further you get from the centre the bigger the area the force is being spread over and the less you feel it.In a true ion the charge will spread out in a perfect sphere.
A negative ion also has a force field but this is caused by the electrons which are able to move. They will move towards to a positive force making the negative ion change shape from a perfect sphere. The further the electrons are from their own nucleus the more easily they are influenced by another positive charge.

So a very small highly charged positive ion creates a very strong positive pull.

A very large negative ion is easily pulled out of shape.

When the two get together the electrons from the negative ion may be so attracted to the positive ion that they get caught up in an orbital of the positive ion. Now they no longer belong exclusively to the negative ion nucleus but are being shared between the two nuclei.
This means there are no longer two separate ions each with their own electrons (ionic bonding) but the two nuclei are both pulling on the same electrons - covalent.

The electrons are  not shared evenly between the two nuclei, they are more attracted to the atom that was the negative ion. The bond is polarised being more negative at one end than the other.

The more protons in the nucleus and the smaller the radius of the atom, the greater attraction it will have for a shared pair of electrons (this is electronegativity)
 

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What happens when cation anion pairs polarise one another?
« Reply #1 on: 17/01/2011 20:30:24 »

 

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