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1. Ok, so CaBr2. I need to draw the dot cross diagram and I know how to draw it but I don't understand why because:

Ca - 2, 8, 10

Br - 2, 8, 18, 7

(according to the 2,8,18,32 rule)

How does that become:

[Ca]2+ [Br]- [Br]- ???

Thanks!
2. You can only fill the 3rd shell with 18 if you have 18 or more electrons left after filling the first two.
If you write down Ca's electron configuration as 2,8,8,2..it's easy to see how it will become 2+ by giving each Br an electron.
3. (Original post by candycloud)
(according to the 2,8,18,32 rule)
If you are going to use simple electron shells, rather than sub-shells, 2,8,8,18 is what you should be using.

TBH, i dont know of any specification that requires you to do electronic configuration of anything larger than Calcium using the dot-cross diagrams. As soon as you get into the transition elements it gets a bit tricky and to truly understand what is going on you need to know about s, p and d shells and their relative energy levels. dot cross diagrams just dont work for Bromine.

Long story short, shell 4 starts to fill up before shell 3 is completely filled, thus Calcium is 2,8,8,2 rather than 2,8,10. You dont need to understand why for GCSE, but you do need to be able to write it.

Simplistically, Bromine is in group 7, thus it requires 1 extra electron to have a stable electronic configuration in the same way as Fluorine and Chlorine (dont worry about what's going on with the rest of it's electrons). If you HAVE to write it, make sure it is 2,8,8,17 as that is what they will be expecting. It's not right but that's life for GCSE chemistry i'm afraid!

Using these configurations, getting the correct ions should be fairly simple.

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Updated: December 24, 2010
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