Filling the levels in order

Showing how the electrons of an atom are filled into the energy levels, in order from the nucleus outwards, is called the electron configuration.

You already have everything needed to work one out. The atomic number gives the number of electrons, and table 3.1 gives how many each level can hold. Fill from the inside, and do not exceed a level's maximum.

Sodium, worked through

• The atomic number of sodium is 11.
• So a sodium atom has 11 protons and 11 electrons.

Now place those 11 electrons, starting at the first level: