Posted by Larry Hoover on April 29, 2009, at 19:34:07
In reply to chemistry help, posted by alexandra_k on April 29, 2009, at 13:19:28
> So I thought I was getting the difference between ionic / extravalent bonds and covalent bonds but now I'm confused again...
Not so confused as you thought, methinks.
You asked:
"Maybe I'm missing some kind of hierarchy to do with the strength of bonds or something like that... "Precisely so. You're asking just the same questions that were in fact answered by Bohr's theory.
> NaCl is an example of an extravalent bond because Sodium DONATES one electron to Clorine to satisfy the octet rule.
> H2 is an example of a single covalent bond because two elements of Hydrogen SHARE one electron (one pair of electrons) to satisfy the octet rule.Minor correction. Two atoms.
> C02 is an example of a double covalent bond because two elements of oxygen SHARE two pairs of electrons with carbon to satisfy the octet rule.
Again, two atoms of oxygen.
> H20 is an example of a double covalent bond because two elements of Hydrogen SHARE two pairs of electrons to satisfy the octet rule.
Again, atoms.
> But then apparently 'water molecules dissociate continually in a reversible reaction to form hydrogen ions (H+ and hydroxide ions (OH-):
>
> H20 <--> H+ + OH-
>
> Hmm.It would be more understandable, perhaps, if you had the correct equation.
2H20 <--> H3O+ + OH-
The electron octets are retained. Hydrogen ions (H+) are actually quite rare, whereas hydronium ions (H3O+) are not.
> But then does H2 similarly dissociate continually in a reversible reaction to form hydrogen ions (H+ and H-)?Statistically, yes it does. But exceedingly rarely, because the hydride ion (H-) is very reactive.
> And then do carbon dioxide molecules dissociate continually in a reversible reaction to form carbon ions and hydrogen ions (C- and 2O+)?
You've got the polarities backwards, and your signs don't add to zero. But it is once again instability = reactivity that would do these rare forms in with incredible speed.
Superoxide ion is O-, whereas an oxygen atom with a double charge is oxide ion.
The key here to understanding the type of bonds that form is the stability after such a bond has formed.
Sodium metal has one extra electron outside of an octet, so it quite happily rids itself of that electron (explosively donating it to water, for example), leaving it with the stable octet, as Na+ ion.
Chlorine gas is stablized as Cl2, because each atom "borrows" an electron from the other, to form octets. As individual atoms, though, they are stabilized by altogether stealing an electron, to give a full octet, as Cl- ion.
So, chlorine can form stable covalent compounds, or can be stable as a free ion. Ionic bonds are only fractionally as strong as covalent bonds.
> If so... Then I lose the distinction between extravalent bonds and covalent bonds (donating vs sharing).
>
> Does this have something to do with problems locating electrons in clouds?No, that's another property of electrons that doesn't literally influence reactivity.
> Is this basically an issue of 'don't worry about it just keep going and learn about carbs / proteins / lipids'?
No, I think it's better to get this idea settled, so you can build upon it.
I'm happy to help with this.
Lar
poster:Larry Hoover
thread:893467
URL: http://www.dr-bob.org/babble/social/20090421/msgs/893492.html