In chemistry, the term period refers to a horizontal row of the periodic table. Elements in the same period all have the same highest unexcited electron energy level or same ground state energy level. In other words, each atom has the same number of electron shells. As you more down the periodic table, there are more elements per element period because the number of electrons allowed per energy sublevel increases.
The seven periods of the periodic table contain naturally-occurring elements. All elements in period 7 are radioactive.
Period 8 consists solely of yet-to-be-discovered synthetic elements. Period 8 is not found on the typical periodic table, but does show up on extended periodic tables.
Significance of Periods on Periodic Table
Element groups and periods organize the elements of the periodic table according to periodic law. This structure categorizes elements according to their similar chemical and physical properties. As you move across a period, an atom of each element gains an electron and displays less metallic character than the element before it. So, elements within a period on the left side of the table are highly reactive and metallic, while elements on the right side are highly reactive and nonmetallic until you reach the final group. The halogens are nonmetallic and not reactive.
The s-block and p-block elements within the same period tend to have different properties. However, d-block elements within a period are more similar to each other.