Jul 7, 2013
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This is just a concept I never have been able to wrap my head around. How can you tell how many covalent bonds an element can form with others? How many it will be left with as lone pairs? Why would it choose to have lone pairs instead of creating a double or triple bond??

I know Group number influences this somehow as well as the octet rule but everytime I feel like I understand it I realize I've been doing it wrong. Please help.
 
Aug 26, 2014
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If you are referring to lewis structures, to find how many covalent bonds an atom "usually" forms, just draw the lewis dot structure for the atom. Obviously, it doesn't always work this way, sometimes you have structures that have to be "fixed" in order to obey the octet rule. Sometimes you also have to adjust this to account for any formal charges, etc. These adjustments have to be taken into account to reflect experimental results. And obviously, lewis structures sometimes aren't enough to explain so you have to jump into VSEPR and MO theory. But generally, atoms like to have a stable configuration which is why they would have lone pairs or creating a triple or double bond. It's not that they "choose" to make double or triple bonds, it just favors the more stable configuration. Again, this is what we see experimentally so we have to account for it
 
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Jul 7, 2013
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Ok yea I think I keep seeing these exception and it throws me off. But one thing I cant understand is that Group II (alkaline earth metals) have 2 valence electrons so basically shouldnt they either have to form TWO bonds to use them or at least make a double bond to use both of them...otherwise there is one extra electron that has been unpaired. I looked up MgO and it seems it uses both electrons (on Mg) to make a single bond with the O and I was confused because I thought each element must give one electron each for a bond so why would Mg give two?
 

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This is just a concept I never have been able to wrap my head around. How can you tell how many covalent bonds an element can form with others? How many it will be left with as lone pairs? Why would it choose to have lone pairs instead of creating a double or triple bond??

I know Group number influences this somehow as well as the octet rule but everytime I feel like I understand it I realize I've been doing it wrong. Please help.
Start drawing Lewis dot structures for the 2nd row elements. When putting in valence electrons, put one on each side, moving around so you have up to 4 single electrons before adding any others.
So B would have 3 single e- dots around it. C would have 4 single e- dots, N has 3 single dots and one pair, O has 2 single dots and 2 pair, F has 1 single dot and 3 pair, Ne has 4 pairs.
All the elements in the group under each of these have the some preferred conformation.
This works for all non-metals.
The number of single e- dots is the number of bonds that element likes to form, and the number of pairs is the lone pairs that it will have in the most stable conformation.

Elements have personalities of a sort, so as to "why they would choose to have lone pairs" (without going into much more complicated explanations) it's because they want it that way. Just accept it and remember their preferences.
Ok yea I think I keep seeing these exception and it throws me off. But one thing I cant understand is that Group II (alkaline earth metals) have 2 valence electrons so basically shouldnt they either have to form TWO bonds to use them or at least make a double bond to use both of them...otherwise there is one extra electron that has been unpaired. I looked up MgO and it seems it uses both electrons (on Mg) to make a single bond with the O and I was confused because I thought each element must give one electron each for a bond so why would Mg give two?
Metals are different. Especially Alkali and Alkaline Earth metals. They just want to get rid of that extra electron or two so that they can pretend to be noble gases. They really love to unload on non-metals and non-metals love to take it. This is the ionic bond.
Covalent bonds rarely happen with metals (esp ones you'll deal with in gen chem). So there aren't often the electron-sharing covalent bonds that you're thinking of.
 
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