[transgalactic]

regarding case B:
http://img525.imageshack.us/my.php?image=84419342em1.gif

before Br has 17 electrons
after the reaction each Br gets 1 electron
so each Br must have 18 electrons

but in the solution
they have only 8

its not 18
??

[Astrokel]

before Br has 17 electrons

huh?

[transgalactic]

Br=[Ar]3d^10 4s^2 4p^5

10+2+5=17

[Astrokel]

Why do you include the 3d electrons? It doesn't mean 3d, 4s, 4p electrons are the valence electrons when written in this form.

[TheJoker]

When it is written in this form though:

elemental Br=[Ar]3d^10 4s^2 4p^5

you can look at the highest energy level (in this case 4) and then get the valence electrons in these orbitals (in this case 7 valence electrons), that is why each bromine gets one electron, to have eight valence electrons and have a stable octet. That is why it shows eight valence and not 18