Electronic Configuration (CIE IGCSE Chemistry: Co-ordinated Sciences (Double Award))

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What is 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 

Rules of electron-shell filling, IGCSE & GCSE Chemistry revision notes

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

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.

Worked example

Draw and write the electronic structure of magnesium. 

Answer:

  • Magnesium has 12 electrons in total.
  • A maximum of two can fit in the first shell and eight in the second shell.
  • The remaining two will occupy the third shell. 

magnesium-atom

  • The written form of this electronic structure is 2,8,2 

Examiner Tip

You need to be able to write the electronic configuration of the first twenty elements and their ions. You may see electronic configurations using full stops or '+' signs instead of commas. You would not be penalised for using full stops.

Electron shells & The Periodic Table

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 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 

Diagram showing the relationship between the electronic configurations

A diagram to show the two ways to represent electronic structure of chlorine

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 

2-1-3-position-of-chlorine

The position of chlorine on the Periodic Table

 

The noble gases

  • In most atoms, the outermost shell is not full
  • These atoms react with other atoms in order to achieve a full outer shell of electrons making 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
    • They then have the electronic structure of a noble gas (Group VIII)
  • All of the noble gases are unreactive as they have full outer shells and are thus very stable

The Periodic Table showing the location of the noble gases

2-1-3-position-of-noble-gases

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

 

Examiner Tip

The electrons in the outer shell are also known as valency electrons. 

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Caroline

Author: Caroline

Expertise: Physics Lead

Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.