# Need help. :(

• September 15th, 2009, 02:13 PM
Ninjarh
Need help. :(
Hi everyone,
i understand i may have posted this topic in the wrong place, but hey!

First day back to school, in sixth form now (lower of course), takin the adjustment from GCSE to AS, really really badly.

So i ask all of you, ' can you help me with my homework? '
Since i don't understand it at all.
Well.. a bit.

I know how to do an electron-dot-cross diagram for an element, and an ion.
It's just, we've been told to do it for a few compounds and as far as my little brain has told me, IVE NEVER LEARNT HOW TO.

So please help, only gonna give you guys the first question though, since i wanna understand how to do it.

If you could, please explain in a lot of detail how to draw it etc, if you can maybe, draw it in paint, save it and upload it to tinypic.com? Then paste the link, i'd fall in love with you if you did.

Here's the first question:
Draw electron dot-cross diagrams for the following ionic compounds.
a) Lithium Hydride (LiH)

And so on..

Thanks in advance, hoping you guys can help me sooo much.
• September 15th, 2009, 03:03 PM
AlexP
How many electrons in the outer shells of a lithium atom and a hydrogen atom? This will be your total. And it makes sense, when you think about bonding (if you've gone over that) that the two electrons will be drawn next to each other, forming a pair. So how do you think you'd draw that?
• September 15th, 2009, 03:20 PM
Ninjarh
Um..

[h] with a dot next to it.
[li] with a dot next to it.
then an arrow.
then [h] with no dots. or two dots.
and then [li] with two dots or no dots.
..wait, idk, in ionic bonds they lose or gain electrons.. so they'd both me 2.8 right?
• September 15th, 2009, 03:32 PM
Ninjarh
Quote:

Originally Posted by Chemboy
How many electrons in the outer shells of a lithium atom and a hydrogen atom? This will be your total. And it makes sense, when you think about bonding (if you've gone over that) that the two electrons will be drawn next to each other, forming a pair. So how do you think you'd draw that?

• September 16th, 2009, 10:57 PM
electricant
Why don't you try doing a google search of "dot cross diagram", then clicking the 'images' link right at the top of the page?

There are plenty of examples for both covalent and ionic compounds right there.
• September 17th, 2009, 05:53 PM
Arcane_Mathematician
iirc it should look something like H:Li right?

It's been a LONG time since my HS chem class...
• September 20th, 2009, 11:41 AM
Luka
[quote="Arcane_Mathematician"]iirc it should look something like H:Li right?

That would best describe a covalent bond. My best guess is that LiH would be predominalty ionic in character? Meaning you should have distinct Li+ and H:- species, in opposition to shared electrons between the two nuclei.
• September 20th, 2009, 04:52 PM
Arcane_Mathematician
I guess I didn't remember correctly :oops: ... Ah well, at least I remembered something from high school chem.
• September 20th, 2009, 06:39 PM
AlexP
If I recall correctly...would it be [Li<sup>+</sup>]:H? Last time I really did this was four years ago. I recall brackets, but I'm questioning whether the charge is inside them or not. fizzlooney or someone, is this correct? I'm not happy that I don't remember it, but I'm not afraid of being wrong. When you're wrong you can be corrected and learn.
• September 20th, 2009, 08:26 PM
farmboy
Can't really remember myself tbh, but if pushed I would say Li:H where one dot is a circle the other an X. Don't really see why there should be a charge on either atom, but I could be very very easily wrong. Wouldn't be surprised at all if the answer was different entirely, which sort of leads onto my next point...........

To the op, if you've grown up enjoying chemistry don't be put off by the lesson in banality that is A-level chemistry. It's clearly designed to give a few of the basics to those people who intend to go on to study for their degree, but it really fails to give any insight into what the subject is all about. If you have an interest in science then the best thing about chemistry is that it covers just about every subject from the biological to the purely physical. I remeber we had an A-level physics teacher who was always complaining because we never did any quantum theory, and I was dissapointed that it was something I'd probably never understand (not that I do now lol) only to find out quantum mechanics is much more important to chemistry than it is to physics, as far as I know, lol.

I quickly learnt that quantum mechanics whilst having many interesting conclusions, is not very interesting to practice. I've gotten sidetracked, lol, but essentially what i was trying to convey is that A-level chemistry is no reflection on modern chemistry, so when it comes to making those future influencing choices at the end of this year make sure to read up on what it is chemists actually do before writing it off. Having recently finished my degree I genuinely can't think of any other subject id have found more fun.
• September 28th, 2009, 09:25 AM
Scifor Refugee
Weird...I don't think I've ever seen people use lewis dot style diagrams for ionic compounds. They're meant for covalent compounds. I don't think you can necessarily satisfy all the rules for drawing them when you're doing ionic compounds, because you'll end up with the wrong number of valence electrons for some or all of the atoms.
• September 28th, 2009, 12:12 PM
Wild Cobra
I never learned the theory that way. I'm more interested in MO Theory. However, is it the same as using the Lewis Structure? Maybe that link will help.
• October 3rd, 2009, 08:49 PM
gottspieler
Quote:

Originally Posted by Chemboy
If I recall correctly...would it be [Li<sup>+</sup>]:H? Last time I really did this was four years ago. I recall brackets, but I'm questioning whether the charge is inside them or not. fizzlooney or someone, is this correct? I'm not happy that I don't remember it, but I'm not afraid of being wrong. When you're wrong you can be corrected and learn.

I've checked and yes, when Li forms a cation (loses electron/s) brackets are used.

Source: http://www.ausetute.com.au/lewisstr.html