Electronic Configurations (Edexcel IGCSE Chemistry: Double 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)
• 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

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

• You should be able to write the electron configuration for the first twenty elements

Electronic configurations of the first 20 elements

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

• 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 

The relationship between the electronic configurations and periodic table

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 position of chlorine on the periodic table

Chlorine is in Group 7, Period 3