The modern periodic table is organized into seven distinct horizontal periods. As you read down the table from top to bottom, each new period gains one additional electron shell. The period number directly tells you the total number of electron shells.
Elements in Period 1 only have one single electron shell. Elements in Period 2 possess two electron shells, and this simple pattern continues downward. This fundamental organization perfectly matches the strict rules of how electrons fill atomic orbitals.
An electron shell must reach a specific capacity before a brand new period can begin. Moving across a single period from left to right reveals highly predictable chemical trends. As you move right across a period, the atomic radius generally shrinks.
At the exact same time, the ionization energy typically increases. These repeating trends occur because the number of positive protons increases. Meanwhile, the main electron shell remains exactly the same.
The extra protons create a stronger pull that draws the electron shell closer to the nucleus. Understanding these horizontal periods helps chemists predict how different elements will behave and react. It explains why elements on the left are reactive metals and elements on the right are nonmetals.