Period (periodic table row)

Electronic structure and why periods exist

Elements in the same period share the same number of electron shells (principal quantum number). As protons are added one by one across a period, electrons fill the available subshells (s, p, d, f) according to quantum rules. These patterns of filling create the repeating or "periodic" variation in properties that gives the periodic table its organizing power: each new period typically begins when a new electron shell starts to be filled.

Trends and characteristics across a period

Across a single period several well-known trends appear. Atomic radius generally decreases from left to right because increasing nuclear charge pulls electrons closer. Ionization energy and electronegativity tend to increase from left to right, while metallic character decreases — elements change from metals on the left to nonmetals on the right. Chemical behavior therefore changes predictably within a period, although local exceptions occur because of subshell arrangements and electron pairing effects.

Lengths of periods and examples

• Period 1: 2 elements (H, He).
• Periods 2 and 3: 8 elements each (s and p blocks filled).
• Periods 4 and 5: 18 elements each (d-block transition metals included).
• Periods 6 and 7: 32 elements each when the f-block (lanthanides and actinides) are counted as part of the row.

These counts reflect the number of electrons that can occupy the available shells and subshells for those principal quantum numbers, and explain why the rows have unequal lengths.

History and practical importance

The periodic arrangement originated in the 19th century when chemists recognized recurring chemical similarities and ordered elements by increasing atomic mass; later work tied the order to atomic number and electronic structure. Today, periods are fundamental to teaching chemistry, predicting reactivity, designing materials and understanding trends in bonding and spectroscopy.

Notable distinctions and modern considerations

When displayed compactly, the f-block (lanthanides and actinides) is often shown below the main table for typographical reasons, but these elements belong to periods 6 and 7. The seventh period contains many synthetic and radioactive elements created in laboratories and particle accelerators. Because each element in a period differs by a single proton, recognizing period membership is a quick way to infer an element's shell structure and compare it with neighbors.

For further reading on the concept and applications of periods within the periodic table, see linked resources: overview, table layout, element list, atomic number, hydrogen and helium.