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Topic: Understanding chemical bonds  (Read 4169 times)

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Offline Sagittarius

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Understanding chemical bonds
« on: October 31, 2013, 05:58:01 PM »
I have 2 questions:
1:
How good my understanding is?
There are 4 substance states.
1) Hot (no bonds)
2) Excited (Bonds are made not with spins, but with electron orbital density)
3) Spin (related to conditions like in biological systems)
4) Cold (Crystals are formed independently on valence)

There are 3 cases for spin bonds:
1) not formed in case of same spins even if orbitals overlap (He+He)
2) formed in case of different spins even if orbitals do not overlap (H+F)
3) formed in case of different spins only if orbitals overlap (Li+H)

2:
I tried to find 2-nd period atom images in wikipedia, youtube, etc.
but found only Bohr models.
In the book

http://www.chemistry.mcmaster.ca/esam/Chapter_6/section_4.html

I saw such an image of BeH2:
http://www.chemistry.mcmaster.ca/esam/Chapter_6/fig6-18.jpg
A pictorial representation of the overlap of two sp hybrid orbitals on Be with H 1s orbitals to form BeH2

It looks like it is easy to portray 2-nd period atoms. Where can I find such images
and how useful they are for understanding atomic structure and bonds?
Can they be used for any modelling and how useful can it be?
Thank you in advance.

Offline Sagittarius

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atom drawing
« Reply #1 on: November 01, 2013, 12:35:20 PM »
I tried to find 2-nd period atom images in wikipedia, youtube, etc.
but found only Bohr models.
In the book

http://www.chemistry.mcmaster.ca/esam/Chapter_6/section_4.html

I saw such an image of BeH2:
http://www.chemistry.mcmaster.ca/esam/Chapter_6/fig6-18.jpg
A pictorial representation of the overlap of two sp hybrid orbitals on Be with H 1s orbitals to form BeH2

It looks like it is easy to portray 2-nd period atoms. Where can I find such images
and how useful they are for understanding atomic structure and bonds?
Can they be used for any modelling and how useful can it be?
Thank you in advance.

Offline magician4

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Re: atom drawing
« Reply #2 on: November 02, 2013, 02:36:37 PM »
the picture shown doesn't show an atom, but a compound

...and that's a very different story from "atom"

Furthermore, to construct a picture of an isolated, single atom, i.e. the orbitals belonging to it's ground state, in vacuum, is either pretty boring (meaning, you would have a hydrogen type orbital arrangement) or confusing (you would have more realistic data from QM calculations, arising more questions than answers to the beginner)
... and in any case pretty meaningless for the understanding of this atom in a for real chemical environment*): allmost all atoms (except of the noble gases, that is) will form molecules where the atoms are hybridized to  a certain degree or another, with various results, depending on their partners**)

These for-real situations aren't easily predicted, and require advanced knowledge to do so, even if you just want ballpark numbers. However, there are some simple models around (like, for example, VSEPR) that might help you with at least a basic understanding of the situation, showing pathways to at least predict the structure of some simple molecules +/- correctly, and hence the hybridization most probably belonging to


regards

Ingo


*)
meaning that high energy conditions with isolated atoms, ions or plasmas are a matter of physics, where chemical properties of the particles become next to meaningless

**)
(a) for example, strontium halogenides  will show the following bond angles in gas phase : SrF2 108° , SrCl2 120° , SrBr2 , SrI2 180°
(values taken from E.Riedel , modern inorganic chemistry , p. 142 )
(b) in contrast to  Berylliumhydride BeH2 (180°) , Bariumhydride BaH2 will show a bond angle of 116°
Inorg.Chem. 1995, 34, 2407
« Last Edit: November 02, 2013, 03:29:21 PM by magician4 »
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Offline magician4

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Re: Understanding chemical bonds
« Reply #3 on: November 02, 2013, 02:48:42 PM »
ref. 1):

(a) I don't agree with your counting of (?substance states) phases
in the classic sense, there are 3: solid, liquid, gas
(in modern physics, there are even more)

(b) I don't agree with your understanding of "bond"
- what do you mean by "spin"? the orientation of the wavefunction?
(hint: "spin" is reserved for something completely different)
- note: there is no such thing like a bond without orbital overlap. Even LiF - which should be the most extreme example for a ionic bond - still has like 25% covalent bond in gas phase

therefore, my impression is that you don't have a good understanding of "bond" yet


ref. 2): see your other posting


regards

Ingo
« Last Edit: November 02, 2013, 03:10:38 PM by magician4 »
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Offline Sagittarius

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Re: Understanding chemical bonds
« Reply #4 on: November 05, 2013, 01:32:18 PM »
Thank you a lot for your answer.
You typed:
(b) I don't agree with your understanding of "bond"
- what do you mean by "spin"? the orientation of the wavefunction?
(hint: "spin" is reserved for something completely different)

I saw this:
http://www.chemistry.mcmaster.ca/esam/Chapter_6/section_3.html

The Quantum Mechanical Explanation of Valency
Helium atoms in their ground state do not form a stable
diatomic molecule. In fact, helium does not combine with
any neutral atom. Its valency, that is, its ability to form
chemical bonds with other atoms, is zero. The electronic
configuration of the helium atom is 1s2(¬), a closed shell
configuration. When two helium atoms are in contact, each
electron on one atom encounters an electron on the other
atom with a parallel spin. Because of the Pauli principle,
neither electron on either atom can concentrate its density
in the region they have in common, the region between the
nuclei. Instead, the density is transferred to the antibinding
regions behind each nucleus where the overlap of the two atomic
density distributions is least. This is the same effect noted
earlier for the approach of two hydrogen atoms with parallel
spins.


http://en.wikipedia.org/wiki/Hydrogen#Elemental_molecular_forms

There exist two different spin isomers of hydrogen diatomic molecules
that differ by the relative spin of their nuclei.[21] In the
orthohydrogen form, the spins of the two protons are parallel
and form a triplet state with a molecular spin quantum number
of 1 (½+½); in the parahydrogen form the spins are antiparallel
and form a singlet with a molecular spin quantum number of 0
(½–½). At standard temperature and pressure, hydrogen gas contains
about 25% of the para form and 75% of the ortho form, also known as
the "normal form".[22] The equilibrium ratio of orthohydrogen to
parahydrogen depends on temperature, but because the ortho form is
an excited state and has a higher energy than the para form, it is
unstable and cannot be purified. At very low temperatures,
the equilibrium state is composed almost exclusively of the para form.

classic sense might be not good enough for bonds.
Obviously there are crystals and plasma.
Substance in plasma state has no bonds.
Li has valency 1 when forms Li2 but also forms bonds with
other 6 Li atoms. If H2 is frozen and compressed it becomes metal with
probably also crystal structure where its valency is not 1 but more.
By orbital overlap I meant something different (my fault)
(it is hard to describe everything precisely and briefly at the same time)
If some atoms are forced to overlap their orbitals by conditions like heat and
pressure ( for example) they form bonds.
 But in other cases it is not
necessary to force atoms to overlap each other
because their orbitals and nuclei attract each other and stick. (H + F)
In the rest of cases even if you force them to stay close orbitals will push
away each other due to different spins like it was described about He.

Offline Sagittarius

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Re: atom drawing
« Reply #5 on: November 05, 2013, 02:49:57 PM »
Thank you a lot for your answer.
In
http://en.wikipedia.org/wiki/Atomic_orbital
orbitals table shows orbital shapes for different electrons.
Does it mean that in order to obtain entire atom I should lay them on
each other or will they repulse each other somehow?

"the picture shown doesn't show an atom, but a compound "

Can I just separate this compound and why not?
Are orbitals like springs?

"(you would have more realistic data from QM calculations,
arising more questions than answers to the beginner)"

Are there any pictures from QM calculations?
I saw AVQM CD in a library. It shows something similar to atoms
but it did not say which ones.
It looks like it was something like this
http://www.uni-graz.at/imawww/vqm/pages/movies2.html
but I could not find those atoms there now.

Where can I find pictures like this one
http://www.chemistry.mcmaster.ca/esam/Chapter_6/fig6-18.jpg
but for BH3, CH4, NH3, OH2, FH?

Offline Arkcon

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Re: Understanding chemical bonds
« Reply #6 on: November 13, 2013, 07:49:48 PM »
Sagittarius: I hope you don't mind my merging your two threads into one, as magician4 said, your topics are much the same.

I'm sorry you haven't had many responses, but again, you at least got one per original thread, that told you that your knowledge of the topic is incomplete. 

Suggestion:  Re-study the various references you've quoted, try to fill the gaps that you can for yourself, then try to ask a question.  One question.  One topic that you don't understand.  Only.  And we'll try to build from there.  As it is, you're just asking for everything.  And we can't do that one forum post at a time.
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

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