Ionic versus covalent

  • #1
I came here from https://www.physicsforums.com/threads/i-dont-understand-how-hcl-is-not-a-ionic-bond.449393/ where replies are no longer allowed. I wanted to reply to suggest a few things:
  • High school students who like chemistry are far smarter than anyone will give them credit for.
  • High school students who don't like chemistry are also far smarter, but they won't show it in chemistry class.
  • If I understand ionic and covalent bonds correctly, it is a largely useless distinction which only serves to confuse the fact that the polarity of atomic bonding falls along a spectrum, not into two neat categories.
I also wanted to ask about something I noticed. The chemical known as urea differs from the medicine known as hydroxyurea because a hydrogen atom is replaced by an OH. In water, H is positive and OH is negative. Connected to the carbon atom in hydroxyurea, I imagine the same is true, but that difference in electronegativity pales in comparison to the bond that the carbon atom makes. The fact that Carbon can be either +4 or -4 depending on which electron shell it tries to complete figures into this question, but I haven't asked it yet because I'm thinking with my keyboard. I hope you don't mind.

Wouldn't it make sense to say a bond is ionic because our analysis of the electron shells suggests that electrons are shifted from one atom to the other in order to make complete shells, but when that isn't the case (H2, O2, N2, CH4 (in which case we avoid the arbitrary choice of completing all the H's outer shells or the C's outer shell)), it's covalent? That seems a much more useful distinction than all the Linus Pauling stuff I read in that other thread. It identifies convention (do we choose to see the bond as completing electron shells?) rather than some imagined facet of reality as the difference between covalent and ionic bonds.

PS With regard to the prefix system, everything is Basic to me. I teach physics to 3-year-olds.
 

Answers and Replies

  • #2
DrDu
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Have a look at Pauli's book "The nature of the chemical bond" (and forget about the pseudo-Pauliism propagated by most books). He has quite some convincing arguments on how to distinguish between covalent and ionic compounds based on the potential energy surfaces crossing or not.
 
  • #3
"[T]he pseudo-Pauliism propagated by most books" is what interests me the most. The widespread misinterpretation of the written word is fascinating, even more than the development of conventions in language that enable us to discuss things like the ways in which (and the fact that) atoms bind to each other. Do you have any theories on why "pseudo-pauliism" comes about and why it is propagated?

Also, am I wrong in representing the difference between covalent and ionic bonds as convention rather than a facet of reality? It seems the usefulness of the idea relates to ions themselves, which are easily created from some molecules (salt) and very difficult to make from other molecules (O2). Hence the idea of a spectrum.

Thanks for the reference in any case!

Dave.
 
  • #4
DrDu
Science Advisor
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Pauli was the only theoretical chemist I am aware off who wrote a book on general chemistry (and a very good one!) and his ideas became very influential in chemical education. However, most chemistry teachers never hear a theoretical chemistry class during their studies and with time, many of Paulis ideas became mutilated being reduced to more and more formalistic schemes which have little to do with reality. On the other hand, some of Paulings ideas have been disproven by computerized quantum chemistry, which was nearly non-existent when Pauli wrote his books.
 

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