How are groups and periods arranged on the periodic table?

The periodic table organizes chemical elements based on their atomic numbers, which represent the number of protons in an atom's nucleus.12

Periods

The horizontal rows of the table are called periods. There are seven periods in total, arranged in order of increasing atomic number. Each period corresponds to the successive occupation of orbitals within an atom's valence shell, with each period representing a successively higher value of n for that shell.31

Groups

The vertical columns of the table are called groups. Under the modern International Union of Pure and Applied Chemistry (IUPAC) system, these are numbered from 1 to 18. Elements within the same group exhibit a family resemblance because they share analogous outermost (valence) electron configurations, which influences the types and numbers of chemical bonds they can form.34