Period (periodic table)

A period is a horizontal row in the periodic table of chemical elements. Elements in the same period share the same number of electron shells in their outermost filled level—a number equal to the period number.

Moving across a row from left to right, the number of protons (atomic number) increases by one at each step. Chemical properties also change accordingly: on the left are reactive metals, in the centre are transition metals and metalloids, on the right are nonmetals and noble gases. Each row ends with a noble gas.

Number of periods

The standard periodic table has seven periods. Period 1 contains only two elements: hydrogen and helium. Periods 2 and 3 each contain eight elements. Periods 4 and 5 each contain eighteen elements. Periods 6 and 7 each contain thirty-two elements, because they include the separate series of lanthanides and actinides.

Difference between period and group

A group is a vertical column; elements in the same group typically have the same number of electrons in their outer shell and thus have similar chemical properties. A period is a horizontal row; elements in the same row have chemical properties that change progressively and are not alike. For example, sodium and potassium belong to the same group and behave similarly; sodium and chlorine are in the same row but have different properties.

Trends across a period

Moving across a period, atomic radius decreases while ionization energy and electronegativity increase. This occurs because nuclear charge increases while the electron shell does not change, pulling electrons more tightly toward the nucleus. These trends are not perfectly smooth; there are small irregularities related to how electron shells fill.