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Topic: Atomic Radii, Charge, and Electronegativity with Ionic Bonds  (Read 3041 times)

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Offline galpinj

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Hi guys,

For a bond to be ionic, it requires that the constituent atoms have very different electronegativities. It would follow that the larger the difference, the greater the strength of the ionic bond; however, this doesn't seem to always be the case. For example, NaCl has a weaker ionic bond than MgS, even though the difference in electronegativites for Na and Cl is greater than that for Mg and S. I've read that MgS has a more powerful bond because it has a greater charge (+2/-2 as opposed to the +1/-1 for NaCl). I've also read that the charge is dependent on radii size (smaller atoms will have stronger bonds). So, clearly, in this example charge seems to trump electronegativity. Is this always the case? Or does greater electronegatvity ever trump differences in charge/radii size?

Thank you!

Offline j.k.11

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Re: Atomic Radii, Charge, and Electronegativity with Ionic Bonds
« Reply #1 on: July 25, 2016, 08:02:50 PM »
When we look at ionic bonds we typically use a simplified version of Coulomb's Law.


Bond strength = (q1 * q2)/r^2

where q = charge and r = size.

From this law we can tell that higher charge and smaller atoms yields stronger ionic bonds. We generally say that charge presides over size in terms of ranking.

However, we look at electronegativities for a different purpose. We classify bonds as nonpolar, polar, and ionic based on their difference in electronegativity. It is more accurate to instead say that compounds have a certain "%ionic character". That is, they act both like a covalent bond and like an ionic bond.

It is important to note that sometimes there are even exceptions to the electronegativity difference rule for classifying bond character. i.e. BeCl2 or BeF2. So these rules aren't set in stone: it is more important to know why the patterns exist.

Offline galpinj

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Re: Atomic Radii, Charge, and Electronegativity with Ionic Bonds
« Reply #2 on: July 26, 2016, 12:24:14 AM »
When we look at ionic bonds we typically use a simplified version of Coulomb's Law.


Bond strength = (q1 * q2)/r^2

where q = charge and r = size.

From this law we can tell that higher charge and smaller atoms yields stronger ionic bonds. We generally say that charge presides over size in terms of ranking.

However, we look at electronegativities for a different purpose. We classify bonds as nonpolar, polar, and ionic based on their difference in electronegativity. It is more accurate to instead say that compounds have a certain "%ionic character". That is, they act both like a covalent bond and like an ionic bond.

It is important to note that sometimes there are even exceptions to the electronegativity difference rule for classifying bond character. i.e. BeCl2 or BeF2. So these rules aren't set in stone: it is more important to know why the patterns exist.

Thank you for the response, that all made perfect sense! So, does electronegativity only provide an idea of whether two atoms are ionic and has zero relation to the actual strength of the bond? It seems a little counter-intuitive; I would expect that two atoms with different electronegativities would be more drawn together. It's hard to believe that, based on the equation, the difference in electronegativity does not actually have an impact on bond strength.

Offline j.k.11

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Re: Atomic Radii, Charge, and Electronegativity with Ionic Bonds
« Reply #3 on: July 26, 2016, 01:24:48 AM »
I guess you could say that higher EN difference would correlate with higher ionic character and therefore stronger bonds. A higher EN difference allows one atom in the bond to pull more electrons towards its electron cloud, changing its charge. Ionic bonds occur because of Coloumbic forces (+/- attraction) so if one atom has more e- the charge difference is higher making the bond stronger.

Offline galpinj

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Re: Atomic Radii, Charge, and Electronegativity with Ionic Bonds
« Reply #4 on: July 26, 2016, 01:25:51 PM »
I guess you could say that higher EN difference would correlate with higher ionic character and therefore stronger bonds. A higher EN difference allows one atom in the bond to pull more electrons towards its electron cloud, changing its charge. Ionic bonds occur because of Coloumbic forces (+/- attraction) so if one atom has more e- the charge difference is higher making the bond stronger.

But in the example listed, MgS has a stronger ionic bond than NaCl even though NaCl have a greater difference in EN; so, obviously, they don't completely correlate. It's clear that EN difference is important in defining what is an ionic bond, but the degree to which EN affects the strength of the bond remains still remain nebulous to me.

Offline j.k.11

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Re: Atomic Radii, Charge, and Electronegativity with Ionic Bonds
« Reply #5 on: July 26, 2016, 01:51:18 PM »
I can't say this with 100% certainty, but I suppose you have to consider EN, size, and charge in conjunction in order to accurately describe ionic strength. At a high school AP level, comparing size and charge is enough; I know my class never talked about EN as well.

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Re: Atomic Radii, Charge, and Electronegativity with Ionic Bonds
« Reply #6 on: July 26, 2016, 03:31:00 PM »
EN difference would tell you which bond is stronger assuming all other things are identical. That's rarely a case, so you have to consider all other things as well.
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Offline Enthalpy

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Re: Atomic Radii, Charge, and Electronegativity with Ionic Bonds
« Reply #7 on: July 27, 2016, 04:06:27 AM »
Some nasty details:
- Beyond the ion diameters, the mode of packing also determines the distances and the number of neighbours.
- Maybe the partial charge and distance would apply better to a gas. In a solid, it's hard to tell whether the electrons are at one nucleus or the other, since they're close to an other.
- q1q2/r2 tells the force (with some constants), q1q2/r the energy.

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