Bonding question

[thechairman]

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What determines the bonding strength in intramolecular bonds?

Is it safe to say that Ionic bonds>Polar Covalent>Nonpolar Covalent, in terms of bond strength.

 

[jochi1543]

Yes, I would say it's safe. Think of melting/boiling points as one of the indications of bond strength. Ionic compounds, such as table salt, melt at extremely high temps (I think it's in the neighborhood of 3000 C for salt), while non-polar covalent compounds like oxygen have extremely low boiling and melting points.

 

[BlackSails]

What determines the bonding strength in intramolecular bonds?

Is it safe to say that Ionic bonds>Polar Covalent>Nonpolar Covalent, in terms of bond strength.

Basically, the more one atom holds the other atom's electron, the stronger the bond. In nonpolar, they share nicely, but in ionic, the anion steals the entire electrion away.

 

[slipstream99]

I think the bond strength should be the other way around. As in: covalent bonds (~80 kcal/mol) are stronger than ionic bonds (~4-7 kcal/mol).

 

[DocWalken]

Ionic bonds are weaker in solution, but much stronger than covalent in the solid state.

 

[werd]

I think the bond strength should be the other way around. As in: covalent bonds (~80 kcal/mol) are stronger than ionic bonds (~4-7 kcal/mol).

this is a point which is easily confused. watch out for ionic bonding vs. ionic interactions. an ionic bond is present for something like NaCl and is damn strong - just try to melt that stuff! an ionic interaction is when something like RNH3+ 'talks' to something like a -OR' in solution, like in protein folding and such. the charges attract eachother and a weak interaction is formed; you can usually break it with a little extra heat, such as denaturing a protein or using PCR to dissociate dna strands.
in short, ionic bonds = strong. ionic interactions = pretty weak.

 

[slipstream99]

Thanks for clearing that up!

I did a little browsing and found a great explanation of the issue: http://www.newton.dep.anl.gov/askasci/chem03/chem03623.htm

The second response (starting with "There are several problems...") is great.

 

[thechairman]

i think people are getting over intermolecular forces and intramolecular forces here.

it's true that nonpolar - nonpolar intermolecular interactions are the weakest, and ionic interation are the strongest, i think it is opposite for intramolecular forces.

ionic bonds are between an atom with high electronegativity and another with low electronegativity, and bond formation involves the formation of tightly atrracted ions. whereas nonpolar covalent bonding involves sharing of electrons and is weaker.

but one that bugs me is that if ionic bonds are stronger, then why are diamonds , and not rock salt the strongest solids?

 

[Yoyomama88]

i believe that a polar covalent bond can sometimes be stronger than an ionic bond depending on the difference in electronegativities....but as a general rule i would say that ionic bonds are strong than colavent bonds.

 

[werd]

i guess the original question was what determines bond strength. basically the two biggest factors are the difference in electronegativity and the size of the atoms. bigger atoms are expected to form weaker bonds, ie NaCl not as strong as LiCl. the electronegativity difference is used to define the "type" of bond, with EN diff > 1.7 (sources vary) defining ionic instead of polar covalent and somewhere around 1 for polar vs. nonpolar covalent. greater EN difference forms stronger (more ionic) bonds and smaller difference forms weaker (more covalent) bonds.

the other way to answer "what determines bond strength?" is to say that the bond strength equals to the decrease in energy acheived by the two atoms in forming the bond. so, say some atoms are decreasing their net energy by 60kcals by forming a bond... that 60 kcals will be the strength of the bond. it sounds sort of circular but it's another way to look at it.

 

[BlackSails]

but one that bugs me is that if ionic bonds are stronger, then why are diamonds , and not rock salt the strongest solids?

In diamonds, the orbitals are delocalized over the entire structure. That makes it very, very stable.