Structure of the Periodic Table

The periodic table is the central organizing framework of chemistry. It arranges all known elements in order of increasing atomic number (the number of protons in the nucleus) so that elements with similar chemical properties fall into vertical columns. This arrangement is not arbitrary; it is a direct consequence of the way electrons occupy energy levels around the nucleus. Because chemical behavior is governed largely by the outermost electrons, elements with the same outer electron configuration behave in comparable ways, and the table makes these repeating patterns visible at a glance.

The horizontal rows are called periods, numbered 1 through 7. The period number tells you the highest principal energy level (the highest value of the principal quantum number $n$) that contains electron