This line chart tracks the number of electron shells (principal energy levels) as atomic number increases. Each new period in the periodic table corresponds to adding another shell.

The stepwise pattern reflects the periodic table structure. Hydrogen and helium use only shell 1. Elements 3-10 add shell 2, 11-18 add shell 3, and so on. This determines atomic radius trends - atoms grow larger down groups but shrink across periods as increased nuclear charge pulls electrons inward.