alexleyenda - 27-4-2015 at 09:34
Hi,
I have some random questions related to bonds on which I would like deep complete answers. These are quite precise questions, so I have not been able
to find the answer to them.
First, why is the triple bond in N2 stronger than the one in ethyne? And don't tell me it's because the dissociation energy is higher in N2, that is a
consequence of the fact that it is stronger, not the cause ! I looked at the orbitals diagrams and I have seen no reasons there too.
2- Why double bonds and triple bonds are difficult to form in the third period and quite impossible in the fourth and higer periods?
3- Why does layers of BN, which have the same structure as graphite, do not conduct electricity while graphite does?
Thank you !
annaandherdad - 27-4-2015 at 10:16
I think I have a partial answer to number 1. The ionization potential of nitrogen is high because the valence 2p subshell is half-filled, which means
that all electrons can be in the same spin state. When the next electron is added (for oxygen), its spin has to be anti-aligned with the electrons
that are already there, and this causes an increase in the atomic energy level and hence a decrease in the ionization potential. See Fig 1 on p. 21
of my notes,
http://bohr.physics.berkeley.edu/classes/221/1112/notes/hart... and the discussion there.
See also the book, Intermediate Quantum Mechanics, by Bethe and Jackiw.
My notes address the question of the Z dependence of the ionization potential of atoms, which is not the same as the strength of bonds (single,
double, triple ...), but the questions are related.
[Edited on 27-4-2015 by annaandherdad]
[Edited on 27-4-2015 by annaandherdad]
alexleyenda - 27-4-2015 at 10:32
I learned something, it is interesting, though it does not explain the difference between the two molecules :p
blogfast25 - 27-4-2015 at 13:38
The Molecular Orbital energies for light diatomic molecules increases (becomes more negative) from H<sub>2</sub> to
F<sub>2</sub>:
http://en.wikipedia.org/wiki/Molecular_orbital_diagram#MO_en...
As you move down within a group, the elements become more electropositive and less prone to creating purely covalent bonding (even the silanes are
much less stable than their corresponding alkanes).
To understand this better you’d need to study Molecular Orbital Theory (not easy, not even in the case of simple H<sub>2</sub>!)
szuko03 - 27-4-2015 at 15:01
^ wow that brings back memories, memories I did my best to forget.
Chemosynthesis - 27-4-2015 at 15:20
Also if you do look into MO theory, note flipping and that nitrogens have un-bonding lone pairs (these ones are not antibonding).
As for BN, there is a greater delocalized electron character than graphite, which allows movement of charge and fields.