Electronic Configuration (Cambridge (CIE) O Level Chemistry): Revision Note

Electronic Configuration

Electronic configuration

• 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 

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)

  • E.g. Carbon has 6 electrons, 2 in the 1st shell and 4 in the 2nd shell

    • 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

The Electronic Configuration of the First Twenty 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.

Electronic Configuration of Ions 

• Ions are formed when an atom loses or gains electrons to become stable 

  • Positively charged ions are formed when an atom loses electrons

  • Negatively charged ions are formed when an atom gains electrons

  • The size of the charge indicates the number of electrons that have been lost or gained

• To find the electronic configuration of an ion: 

  • Identify the electronic configuration for the atom

  • Identify whether it has lost or gained electrons from its charge

  • Add or remove electrons depending on the charge of the atom 

Electron Shells & 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 will show the number of occupied shells of electrons the atom has, showing the period in which that element is in

• The last notation shows the number of outer electrons the atom has, showing the group that element is in (for elements in Groups I to VII)

• Elements in the same group have the same number of outer shell electrons 

The electronic configuration for chlorine

Period: The red numbers at the bottom show the number of notations which is 3, showing that a chlorine atom has 3 occupied shells of electrons and is in Period 3

Group: The final notation, which is 7 in the example, shows that a chlorine atom has 7 outer electrons and is in Group VII

The position of chlorine on the Periodic Table

• In most atoms, the outermost shell is not full and therefore these atoms react with other atoms in order to achieve a full outer shell of electrons (which would make them more stable)

• In some cases, atoms lose electrons to entirely empty this shell so that the next shell below becomes a (full) outer shell

• All elements wish to fill their outer shells with electrons as this is a much more stable configuration

The noble gases

• The atoms of the Group VIII elements (the noble gases) all have a full outer shell of electrons

• All of the noble gases are unreactive as they have full outer shells and are thus very stable

The noble gases are on the Periodic Table in Group 8/0