I missed school today so the hw is a bit confusing and I have a quiz on this tmr. For example I have Br which has 35 electrons.

Question

[Ar]_______
and then what do I do? How do I know what to put next?
how would you do Phosphorus?
thanks!

Answer 1

I can't draw these on this board but here is what you do. Write the levels this way.
1s
2s 2p
3s 3p 3d
4s 4p 4d 4f
5s 5p 5d 5f
6s 6p 6d 6f
7s 7p 7d 7f
8s etc.
Here is a site that show how these are filled. You simply follow the arrows that start at the top and slant downward. When you come to the bottom of the arrow go back to the top with the next arrow down and follow that. So you fill 1s first, (that's the end of the first arrow so go back to the start of the next arrow which starts at 2s. That's the end of the second arrow; go back to the top of the next downward arrow which starts at 2p. Fill 2p then 3s. Continue this until all is filled. There some exceptions to the mnemonic but we won't worry about that right now. For Br, you write [Ar] which gets you to 1s2 2s2 2p6 3s2 3p6.
So you know you must add another 17 electrons so we add them as 3d10 4s2 4p5 so you write [Ar] 3d10 4s2 4p5. If you want to check yourself you can go to webelements.com, click on the element, scroll down to electron configuration and read it from the board. That has all of the elements done that way. Here is the link for Br
http://www.webelements.com/bromine/atoms.html

Answer 2

thank you! I think I kinda understand it now!