Ionic character in covalent bonds - a question!

[RyanJ]

Hi there everyone!

 

I know that is you take say Caesium Fluoride, even though this substance actually formcs covalent bonds the difference of the electronegativity of the elements ([math]E_d = |(x_a - x_b)| = |(0.7 - 4)| = |(-3.3)| = 3.3)[/math] means that it actually has more ionic character than covalent [math](E_d > 2)[/math] and so is classed as such.

 

My question is how would you work this out for molecules with more than two atoms / molecules bonded together? Say with something like Potassium Nitrate, would you need to work out each bond in this seperatly or is there a better way to do this?

 

Cheers,

 

Ryan Jones

[woelen]

It is a matter ok knowing. You can assume that all ammonium compounds and alkali metal compounds are purely ionic, the compounds of the heavier earth alkali metals (Ca and further down) also can be regarded ionic. For the transition metals and the other metals, it depends on the counter atom/ion.

 

Hydrated chlorates, perchlorates, nitrates and sulfates also may be regarded ionic in all cases. The anhydrous nitrates (e.g. Cu(NO3)2) can be covalent.

 

Another rule of thumb is that the higher the oxidation state of a central element, the more covalent its compounds:

 

E.g. CrO is purely ionic. Cr2O3 is intermediate, although it still can be considered ionic, CrO2 also is intermediate, and not purely ionic anymore. CrO3 is purely covalent. A similar thing holds for Mn in all its oxidation states.

 

Also keep in mind, that ions themselves mostly are covalent entities. E.g. for KNO3, the salt is ionic, K(+) and NO3(-), but the ion NO3(-) itself is purely covalent, there is no nitrogen ion and oxide ion in this.

[RyanJ]

Ah, I see - thanks woelen.

 

So if you know the type of the compound, e.g. chlorate you can probably say its ionic? Thanks

 

Cheers,

 

Ryan Jones