How many periods are in the periodic table?Each horizontal row of elements in the Periodic Table is known as a period. The Periodic Table consists of seven periods, from Period 1 to Period 7. Table shows the changes in the proton numbers and number of valence electrons when going across Period 2.Element in Period 2 Li Be B C N O F NeProton … [Read more...] about How many periods are in the periodic table?