ELECTRON SHELLS

ELECTRON SHELLS

1. ELECTRON SHELL RULES

   • 1. electrons occupy shells - sometimes called energy levels
     • 2. the lowest energy levels are always filled first
       • 3. only a certain number of electrons are allowed in each shells

2. WORKING OUT ELECTRONIC STRUCTURES

   • the electronic structure for the first 20 elements are
     • they are not hard to work out for example nitrogen has 7 protons and so it must have 7 electrons -
       • following the electron rule that the first shell can only take 2 electrons and that the seconds and third can take 8
         • so 2 in the first shell and the remaining 5 in the second shell making it
           • 2:5
             • so to calculate the structure of argon
               • its got 18 protons and so it must have 18 electrons
                 • the first shell must have 2 electrons and the second shell must have 8 and as there are 8 left they would fill the next shell making the electron configurations 2:8:8
                   • you can also work out the electron configuration of an element from its period and group
                     • THE NUMBER OF SHELLS WHICH CONTAIN ELECTRONS ARE THE SAME AS THE PERIOD OF THE ELEMENT
                       • THE GROUP NUMBER TELLS YOU HOW MANY ELECTRONS OCCUPY THE OUTER SHELL OF THE ELEMENT
                         • EG-- sodium is in PERIOD 3 , so it has 3 SHELLS OCCUPIED, - so the first two shells must be full 2:8:-
                           • its in GROUP 1 so it has 1 ELECTRON in its outer shell so its electronic structure would be
                             • 2:8:1

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