Shells and the periodic table

In the Bohr model, electrons fill shells from the inside out. The first shell holds at most 2 electrons; the second holds up to 8. Lithium (3 electrons) is 2, 1; neon (10 electrons) is 2, 8.

The periodic table follows the same pattern. The atomic number (proton count) gives the element its box. The period (row) is the number of occupied shells in the neutral atom. For these main-group elements the outer-shell electron count matches the group: 1 in group 1, 2 in group 2, and 3 to 8 in groups 13 to 18. Helium is the exception: group 18, but its only shell is full with 2.