[EvilFrog (OP)]

http://what has electronegativity  got to do with polarisation?

[DrChemistry]

By polarisation, you mean the polarity of a bond or molecule?


Electronegativity, is the 'capability degree' of an atom to attract electrons from other atoms in a bonded molecule. We usually use the pauling scale for that.

Lets take the best know Polar solvent. Water.

Imagine the water molecule:
[diagram=540_0]

Oxygen is more electronegative than hydrogen, and it will start pulling the electrons from hydrogen towards itself, forming a polar covalent bond.  You can imagine, if Oxygen absorbs more electrons, it'd become more negative, the others slightly positive!

We show this like so:
[diagram=541_0]

More or less. I should practice keeping my hand steady.[]

[EvilFrog (OP)]

Haha! Thanks for try so hard to show me.. thanks again...
If the different in the electronegativity between the ions in a compound is small, the polarisation is big or small?
Polarisation cause the partially covalent properties of an ionic compound is it?
If yes for both questions above, then how come the different in electronegativity between Ag ion and Cl ion is small, but there is partially covalent properties between them?

[DrChemistry]

If the different in the electronegativity between the ions in a compound is small, the polarisation is big or small?

The smaller difference in electronegativity, the less polar the bond is. When then difference is very small 0-0.1, we usually do not make a mistake referring to it as a non-polar covalent bond.

P.s. I see other people saying that 0-0.4 is the range for Non-polar covalency. 0.4-1.7 is the polar covalent range. Above 1.7 is the Ionic range.

Polarisation cause the partially covalent properties of an ionic compound is it?

I would rephrase that. A Covalent molecule or compound has covalent character (hehe, that sounds logical, right?). Although many compounds dissociate in water, it does not necessarily make it an ionic compound.  Take HCl for instance. It dissociates like so in water:

HCl + H₂O  ↔  H₃O⁺ + Cl^-

Or well, more famously anyway:

HCl ↔ H⁺ + Cl^-

It dissociates in water forming ions, however the HCl molecule is a Polar Covalent one. The difference in electronegativity is 1, so we can classify it as Polar Covalent.

Then there are salts and metal hydrides. These typically display ionic character. NaCl has a difference of 2.1 and is hence an Ionic compound with properties following that (Usually higher b.p., better solubility in a polar solvent and better electricity conductor).



Change in properties do occur. So it is perfectly possible, if that was what you referred to in this question, to have a seemingly ionic molecule displaying polar covalent character. One such molecule could be HF, which despite its 1.9 difference in electronegativity is actually a polar covalent molecule.

If yes for both questions above, then how come the different in electronegativity between Ag ion and Cl ion is small, but there is partially covalent properties between them?

I wouldn't say the difference is so small. The difference in electronegativity according to my table of electronegativity is 1.3, so this molecule should show polar covalent properties (e.g. low solubility in water).


You will find there are quite a few exceptions to these rules. Its not always easy finding you way trough the world of covalency  []