If groups are the vertical columns of the periodic table — families of elements with similar personalities — then periods are the horizontal rows, each one a journey from left to right across the chemical landscape. There are seven periods, and the number of each period tells you exactly how many electron shells its elements possess. Period 1 elements (hydrogen and helium) have one shell. Period 4 elements like potassium, iron, and bromine all have four shells. It is a beautifully simple organizing principle.
What makes periods fascinating is the story they tell as you read them left to right. Each period begins with an eager alkali metal (except period 1, which starts with hydrogen) that can barely hold onto its single outer electron, and ends with a noble gas whose outer shell is comfortably full. In between, you travel through a dramatic transformation: metallic character decreases, electronegativity increases, atomic radius shrinks, and ionization energy rises. Sodium on the left of period 3 is a soft, reactive metal that explodes in water; chlorine on the right is a toxic, electron-hungry gas; and argon at the far end wants nothing to do with anyone. Same period, wildly different chemistry.
Periods also grow longer as you go down. Periods 1 has just 2 elements, periods 2 and 3 have 8 each, periods 4 and 5 contain 18, and periods 6 and 7 stretch to 32 (with the lanthanides and actinides usually displayed separately to keep the table manageable). This expansion reflects the filling of new subshells — d orbitals in periods 4 and 5, f orbitals in periods 6 and 7 — adding the transition metals and rare earths that give us everything from surgical steel to smartphone magnets.