How Many Periods Are in the Periodic Table? (7 Explained)

The periodic table is organized into seven horizontal rows called periods. The answer to “how many periods are in the periodic table” is seven. Each period corresponds to the number of electron shells an atom of an element in that row possesses, and the properties of elements shift predictably from left to right across a period.

How Periods Work

A period starts with an element that has one electron in a new outer shell (typically an alkali metal) and ends with a noble gas, which has a full outer shell. As you move across a period, atomic radius generally decreases because the increasing nuclear charge pulls the electron cloud inward. This affects other properties like ionization energy and electronegativity, which tend to increase across the period.

For example, Period 2 begins with lithium (atomic number 3) and ends with neon (atomic number 10). Lithium has a single electron in its second shell, while neon has a full set of eight electrons in that shell. The elements in between—beryllium, boron, carbon, nitrogen, oxygen, and fluorine—show a gradual shift from metallic to non-metallic character. The same pattern repeats in each of the seven periods, though the later periods include more elements because they fill d and f subshells.

The first three periods are short (2, 8, and 8 elements respectively), while periods 4 and 5 each contain 18 elements. Period 6 has 32 elements because it includes the lanthanide series, and Period 7 is also a 32-element row that contains the actinide series. The International Union of Pure and Applied Chemistry (IUPAC) defines the standard periodic table with these seven complete periods.

Key Takeaways

• The periodic table has seven periods, each representing a row where elements share the same number of electron shells.
• Period 1 has 2 elements (hydrogen and helium); periods 2 and 3 have 8 each; periods 4 and 5 have 18 each; periods 6 and 7 have 32 each (including the lanthanides and actinides).
• Moving left to right across a period, atomic radius shrinks while ionization energy and electronegativity generally increase.
• The Royal Society of Chemistry maintains an interactive periodic table that shows each period’s elements and their properties.