Working out the electronic configuration of an atom can be done in several ways, although the method I'll be demonstrating below is the fastest. If you have time at the end to check, I would recommend adding up the numbers of electrons to see if it is correct.
To use this method, you must first understand the periodic table is arranged in a way based upon electrons.
Elements in the periodic table are arranged into groups based upon reacting in similar ways to many other elements. This depends on the number of electrons in the outer shell. Therefore, the group number, or columns running vertically, of an atom tells you how many electrons in the outer shell*.
Elements are also arranged into rows running horizontally. The row number of an atom tells you how many shells there are.
How many electrons are in a full shell?
- 1st shell - 2 electrons.
- 2nd shell - 8 electrons.
- 3rd shell - 8 electrons.
Many GCSE syllabi expect you to be able to work out the electron configuration for atoms up to and including calcium (atomic number 20) and this has 2 electrons in the 4th shell. Beyond here, things get a little more complex*.
So what if you forget how many electrons are in a full shell?
The periodic table can tell you this too! Atoms in group 8/0, the noble gases, have full shells of electrons; this is why they are so unreactive. Helium has 2 electrons, neon has 10 and argon has 18. Therefore the first shell can hold 2 electrons, the second shell can hold 10-2 = 8 electrons and the final shell can hold 18 - (10+2) = 8 electrons.
So, there you have it! With a little practise, this will make electron configurations far easier.
Carbon: Group 4, row 2. It has 4 electrons in the outer shell and 2 shells in total. Configuration would be 2, 4.
Chlorine: Group 7, row 3. 7 electrons in the outer shell and 3 shells in total. Configuration would be 2, 8, 7.
Potassium: Group 1, row 4. 1 electron in the outer shell and 4 shells in total. Configuration would be 2, 8, 8, 1.
- Row number = number of shells (don't forget hydrogen and helium count as row 1!).
- Group number = number of electrons in the outermost (valence) shell.
- Shell 1 can hold 2 electrons at most.
- Shells 2 and 3 can hold 8 electrons at most (also called the octet rule).
*When I mentioned things can get complicated, this is something for A-level. If you are really interested the shells can be split into different sections called sub-shells, orbitals, etc. An AS chemistry textbook would help explain it (wikipedia is very heavy for learning this first-time round!)