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Topic: Lithium forms covalent Halides. Why?  (Read 15576 times)

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Offline xstrae

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Lithium forms covalent Halides. Why?
« on: August 02, 2006, 11:21:34 AM »
Why cant the 2s1 electron be transfered to Fluorine and hence an ionic bond formed??  ???Can someone please explain?

Offline Albert

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Re: Lithium forms covalent Halides. Why?
« Reply #1 on: August 02, 2006, 11:31:06 AM »
I would say it's definitely an ionic bond. http://www.chemistry.mcmaster.ca/esam/Chapter_7/section_1.html

 ???

Offline xstrae

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Re: Lithium forms covalent Halides. Why?
« Reply #2 on: August 02, 2006, 11:58:22 AM »
no, my tutor and various sources I have looked up say that Lithium forms covalent halides.

Offline xiankai

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Re: Lithium forms covalent Halides. Why?
« Reply #3 on: August 02, 2006, 08:18:59 PM »
this is very interesting. i've been reading about it as an example of polar covalence character in supposedly 'ionic' compounds in an online textbook.

http://www.chem1.com/acad/webtext/chembond/cb04.html

apparently the answer lies somewhere in the small size of the atoms. the small atomic radii leads to lithium not completely 'losing' its electron to fluorine, because the nucleus still attracts it with a strong force. in fact, the average distance from the 'lost' electron to the lithium nucleus is actually smaller than the atomic radii of lithium! (156pm vs 252pm)

in actuality, ionic bond and covalent bond have alot in similarity. ionic bond involves an atom giving up electrons to another atom. but how are they attracted? of course! the nucleus of the cation still is attracted to the electrons that it 'lost'. otherwise, there would be no way for it to be attracted to the anion. likewise, there is attraction from the nucleus of the anion to the electrons of the cations. this makes it seem a covalent bond indeed. at the very fundamental level there is little or no difference. the difference lies mainly in our perceptions of it.

the online textbook also provided a reference, you may be interested in checking it out:

R.T. Sanderson: Chemical bonds and bond energy, Chapter 9 (Academic Press)
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Offline Mitch

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Re: Lithium forms covalent Halides. Why?
« Reply #4 on: August 02, 2006, 10:53:39 PM »
So if you stick LiCl into deionized water, the resulting water mixture will not be able to conduct a charge?
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Offline xiankai

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Re: Lithium forms covalent Halides. Why?
« Reply #5 on: August 03, 2006, 03:45:51 AM »
from what i can tell; it still should be able to, given that a polar bond still exists.
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Re: Lithium forms covalent Halides. Why?
« Reply #6 on: August 03, 2006, 04:30:48 AM »
All: have you ever seen "pure ionic" bond? Every bond is covalent to some extent, we are talking about continuum, not separate sets.
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Offline Albert

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Re: Lithium forms covalent Halides. Why?
« Reply #7 on: August 03, 2006, 04:34:12 AM »
That's why I would call this an ionic bond.

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Re: Lithium forms covalent Halides. Why?
« Reply #8 on: August 03, 2006, 05:02:04 AM »
Sometimes ionic component is dominating, sometimes covalent component is dominating. It depends on the difference in electronegativities:

% = 16*D + 3.5*D2

where D is absolute value of electronegativities difference. Difference is close to 3 in this case, so the bond should be about 80% ionic.

But, as xiankai pointed out, in the case of light elements things are not that simple. Read text at the link he provided. IMHO problem is our definitions are a little bit too general and in the border cases lack of precision starts to hunt us :)
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Offline xstrae

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Re: Lithium forms covalent Halides. Why?
« Reply #9 on: August 03, 2006, 08:29:30 AM »
My professor says in LiF and other halides of Li, the covalent character is predominant because of these factors:

1. Small size of Lithium atom
2. Large Charge to Mass ratio
3. High electronegativity of the halogen.

But i dont understand understand the second point. Can you please explain?

Offline xiankai

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Re: Lithium forms covalent Halides. Why?
« Reply #10 on: August 03, 2006, 09:23:01 AM »
apparently the answer lies somewhere in the small size of the atoms. the small atomic radii leads to lithium not completely 'losing' its electron to fluorine, because the nucleus still attracts it with a strong force. in fact, the average distance from the 'lost' electron to the lithium nucleus is actually smaller than the atomic radii of lithium! (156pm vs 252pm)

since the net charge of the Li+ is +1, the same as Na+ and K+, you could say they have the same charge.

since lithium has the smallest mass, it has a relatively high large charge to mass ratio. which leads to the effect as i explained earlier.
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Offline xstrae

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Re: Lithium forms covalent Halides. Why?
« Reply #11 on: August 04, 2006, 03:52:23 AM »
ok thanks a lot!  :)

Can you clear this doubt also?
For cations of the same size and charge, the one with electronic configuration (n-1)dnnso, typical of transition metals, is more polarising than the one with a noble gas configuration, ns2np6, typical of alkali and alkaline earth metal cations.
Why is this so?

Offline xiankai

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Re: Lithium forms covalent Halides. Why?
« Reply #12 on: August 04, 2006, 07:40:23 AM »
although i have never heard of this before, i find it somewhat ambiguous; what do you define as same size?

my take on this is that the smaller the cation (in terms of ionic radii and no. of electron shells), the stronger the polarity. this is as the electrostatic attraction at shorter distances is stronger. with less electron shells there is also less electron shielding.

since n ? 4 as smallest d subshell is 3d, note that the noble gas configuration is usually larger (in the terms i defined above) than the electronic configuration that u specified, in terms of quantum number and subshell (4s2 4p6 vs 3d4 4s0 for example). based on this, we can infer too that the cation with the noble gas configuration has more protons than the E.C. specified, since both have same charge and hence lost same number of electrons.
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Offline stoneburner

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Re: Lithium forms covalent Halides. Why?
« Reply #13 on: August 06, 2006, 05:03:57 PM »
Calculations about % ionic or covalent character are one thing - and tell you what the pure solid state electron distribution is like. Nevertheless, it certainly doesn't ACT like a compound that is fully covalent. Compare LiF's physical properties with that of, say, BF3. LiF melts at 845 Celcius, BF3 is a gas condensing to a liquid at - 100 C, and freezing at - 127 C. (Yes, those are minus signs, not dashes).
 

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Re: Lithium forms covalent Halides. Why?
« Reply #14 on: August 07, 2006, 01:34:41 AM »
LiF crystallizes in space group Fm-3m (the same as NaCl) with octahedral coordination around both Li+ and F-, and is evidently ionic in the solid state.
More complex lithium compounds tends to form Li2X2 cordination patterns (rectangular) with other complexed species (eg water, methanol and so on)
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