Electronic Structure
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
- E.g. Carbon has 6 electrons, 2 in the 1st shell and 4 in the 2nd shell
- 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
- 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
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.
- The written form of this electronic structure is 2,8,2
Examiner Tip
It is a good idea to draw the electrons in their shells in pairs.
You will still score the marks if they aren't, as long as you have the correct number in each shell, but this makes it easier for the examiner to count.
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
Examiner Tip
The group number will be labelled on the Periodic Table you are given in your exam, but the period number isn't so it is a good idea to write this on yourself at the beginning.