The meaning of the word period is repeating at a regular interval or repeating in the same order For Example: In periodic table atomic number always increased by 1 when we go left to right in a row , where as , as we go from top to bottom in a group of periodic table atomic number increases with regular repeating interval of (2,8,8,18,18,32).Another example of atomic radius is when we go from top to bottom in a group atomic radius increases.