Answer;
-Period
Explanation;
-Properties of elements within a period on the periodic table change in a predictable way from one side of the table to the other. A period in the periodic table is a horizontal row. All elements in a row have the same number of electron shells.
-Elements in the same period have the same number of electron shells; moving across a period, elements gain electrons and protons and become less metallic.This arrangement reflects the periodic recurrence of similar properties as the atomic number increases.