Electronic Structure
How the Periodic Table links to Electronic Structure
- There are over 100 chemical elements which have been isolated and identified
- Elements are arranged on the periodic table in order of increasing atomic number
- Each element has one proton more than the element preceding it
- This means that each element also starts with one electron more than the element preceding as a neutral atom starts with a number of electrons equal to the number of protons
- The table is arranged in vertical columns called groups and in rows called periods
- Periods:
- These are the horizontal rows that show the number of shells of electrons an atom has
- They are numbered from 1 - 7
- For example, elements in Period 2 have two electron shells, elements in Period 3 have three electron shells
- Groups:
- These are the vertical columns that show how many outer electrons each atom has
- They are numbered from 1 - 7, with a final group called group 0 (instead of Group 8)
- For example, Group 4 elements have atoms with 4 electrons in the outermost shell, Group 6 elements have atoms with 6 electrons in the outermost shell
- Many groups also have alternative names. Group 1 is also known as the 'alkali metals', Group 7 is also known as the 'halogens' and Group 0 is also known as the 'noble gases'
- Periods:
Electron Shell Diagrams
The electron shells
- We can represent the electronic structure of atoms using electron shell diagrams
- Electrons orbit the nucleus in shells and each shell has a different amount of energy associated with it
- The further away from the nucleus, the more energy a shell has, and the harder the shell is to fill
- Electrons first occupy the shell closest to the nucleus which can hold a maximum of 2 electrons
- When a shell becomes full of electrons, additional electrons have to be added to the next shell
- The second shell and third shell can hold a maximum of 8 electrons each
- 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
- In most atoms, the outermost shell is not full. This means that these atoms react with other atoms in order to achieve a full outer shell of electrons in order to make them more stable
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- Atoms can gain electrons in order to complete their existing outer shell - non-metals do this - to form anions
- Atoms can lose electrons to entirely empty this outer shell so that the next shell below becomes the (full) outer shell - metals do this - to form cations
- Atoms can also share electrons to gain full outer shells - to form covalent bonds
Examiner Tip
All of the shells of electrons up to the outer shell will be full when you draw or write electronic structures.
If you are drawing 1, 2, 3 or 4 electrons you should spread them out with one per quarter of the shell. You can draw the electrons in pairs or equally spread out if you are drawing 5, 6, 7 or 8 electrons in the shell. You can use dots or crosses to show an electron, but you must make sure they are on the line of that shell and not floating between shells.
You can use the term ‘shell’ or ‘energy level’ to describe the space that electrons occupy.
Electron Configuration - Number Notation
- The arrangement of electrons in shells can also be explained using numbers
- This method consists of writing down the number of electrons in each shell and separating each number with a comma or full stop, starting with the innermost shell that is closest to the nucleus
- There is a clear relationship between the number of outer shell electrons and the group number - Group 2 elements have 2 electrons in the outer shell
- How many numbers are present in the electronic configuration will show the number of shells of electrons the atom has, showing the period the element is in - 2.8.2 would mean that the element was in Period 3 (and Group 2)
- Elements in the same group have the same number of outer shell electrons
- Therefore electronic structure of 2.2 and 2.8.2 and 2.8.8.2 all show Group 2 elements as they all end in '2'
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 shells of electrons
- Group: The last notation, in this case 7, shows that chlorine has 7 outer electrons, and therefore is in group 7
The position of chlorine on the Periodic Table
Electronic Configuration of the First 20 Elements Table
Examiner Tip
Although the third shell can actually 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 final part of the third shell.
This means that the maximum of 8 electrons in shell 3 rule applies for all questions for this specification.
