## Working out the electronic arrangement from the atomic number

The atomic number of an atom is the number of protons and this is the same as the number of electrons. Thus, if from the atomic number of an element, its electron arrangement can be determined.

Remember, for the first 20 elements:

• The first shell (the one nearest to the nucleus) can hold just two electrons.
• The second shell can hold up to 8 electrons.
• The third shell can also hold up to 8 electrons.

### Example: Silicon

1. The atomic number of silicon can be determined by finding the element on the Periodic Table. The atomic number of silicon is 14.
2. Start filling the innermost shell and working outwards. Remember the rule about the maximum number of electrons any specific shell can hold.
3. Add two electrons to the innermost shell, this leaves 14−2 = 12 electrons.
4. Add eight of the remaining 12 in the second shell leaving 12−8 = 4 electrons.
5. The remaining 4 electrons are added on the third/outermost shell. So silicon has the electronic structure 2.8.4