Electronic configurations
First shell: maximum of two electrons
Second shell: maximum of eight electrons
Third shell: maximum of eight electrons
(OK it gets more complicated at A-level but save it for now!!)
So, if you have 11 electrons to deal with (like sodium): 2,8,1
And if you have 19 electrons (like potassium) to deal with: 2,8,8,1
The “1” in the outside shell is common to all of Group I.
Similarly, if you have 9 electrons to deal with (like fluorine): 2,7
And if you have 17 electrons (like chlorine) to deal with: 2,8,7
The “7” in the outside shell is common to all of Group VII.
GCSE expects you to go up to calcium (2,8,8,2). Beyond that, the A-level complications get in the way. Transition metals extend their third shell to hold up to 18 altogether. These complications help to explain why bromine doesn't seem to have 7 electrons inthe outside shell if you just count using the GCSE rules. If you are scared by this, don't be. If you are fascinated by it, sign up for A-level Chemistry!!!
