Electronic Structure (AQA GCSE Chemistry: Combined Science)

• We can represent the structure of the atom in two ways: using diagrams called electron shell diagrams or by writing out a special notation called the electronic configuration (or electronic structure or electron distribution)

Electron shell diagrams

• Electrons orbit the nucleus in shells (or energy levels) and each shell has a different amount of energy associated with it
• The further away from the nucleus, the more energy a shell has
• Electrons fill the shell closest to the nucleus 
• When a shell becomes full of electrons, additional electrons have to be added to the next shell
  • The first shell can hold 2 electrons
  • The second shell can hold 8 electrons 
• For this course, a simplified model is used that suggests that the third shell can hold 8 electrons
  • For the first 20 elements, once the third shell has 8 electrons, the fourth shell begins to fill
• The outermost shell of an atom is called the valence shell and an atom is much more stable if it can manage to completely fill this shell with electrons 

Electron shell diagram

A simplified model showing the electron shells

• The arrangement of electrons in shells can also be explained using numbers
• Instead of drawing electron shell diagrams, the number of electrons in each electron shell can be written down, separated by commas
• This notation is called the electronic configuration (or electronic structure)
  • The electronic structure of carbon is 6 electrons, 2 in the 1st shell and 4 in the 2nd shell
    • So its electronic configuration is 2,4
• Electronic configurations can also be written for ions
  • E.g. A sodium atom has 11 electrons, a sodium ion has lost one electron, therefore has 10 electrons; 2 in the first shell and 8 in the 2nd shell
    • Its electronic configuration is 2,8

How does the electronic structure of an element relate to its location in the Periodic Table?

• There is a clear relationship between the electronic configuration and how the Periodic Table is designed
• The number of notations in the electronic configuration tells us the number of occupied shells
  • This tells us what period an element is in
• The last notation shows the number of outer electrons the atom has
  • This tells us the group an element is in
• Elements in the same group have the same number of outer shell electrons 

Diagram showing the relationship between the electronic configurations

The electronic configuration for chlorine

• Period: The red numbers at the bottom show the number of notations 
  • The number of notations is 3
  • Therefore chlorine has 3 occupied shells 
• Group: The last notation, in this case 7
  • This means that chlorine has 7 electrons in its outer shell 
  • Chlorine is therefore in Group 7

The Periodic Table showing the location of chlorine 

Chlorine is in Group 7, Period 3

The Electronic Configuration of the First Twenty Elements

Element	Atomic Number	Electronic Configuration
hydrogen	1	1
helium	2	2
lithium	3	2,1
berylium	4	2,2
boron	5	2,3
carbon	6	2,4
nitrogen	7	2,5
oxygen	8	2,6
fluorine	9	2,7
neon	10	2,8
sodium	11	2,8,1
magnesium	12	2,8,2
aluminium	13	2,8,3
silicon	14	2,8,4
phosphorus	15	2,8,5
sulfur	16	2,8,6
chlorine	17	2,8,7
argon	18	2,8,8
potassium	19	2,8,8,1
calcium	20	2,8,8,2

Note: although the third shell can hold up to 18 electrons, the filling of the shells follows a more complicated pattern after potassium and calcium. For these two elements, the third shell holds 8 and the remaining electrons (for reasons of stability) occupy the fourth shell first before filling the third shell.