A period is the name given to a horizontal row of the periodic table.

The periodic table has seven periods. Period 1 contains only two elements: hydrogen and helium. Period 6 (including the lanthanides running from lanthanum to lutetium) contains 32 elements and so is home to more elements than any other period.

Each period begins with an alkali metal and ends with a noble gas, though period 7 is not complete; discovery claims for the putative noble gas with 118 protons have not yet been accepted.

Search the Dictionary