If you are eligible for reinstatement and wish to return to federal service the reinstatement program affords you an advantage other applicants don't have. You will have to locate openings and at a grade at or below the grade you left federal service. The process is straight forward and outlined below.
The period number is an identifier used to describe the row of the standard periodic table in which the element appears. The lanthanoids and actinoids are extensions of Periods 6 and 7 respectively, hence their assignment to those Periods.
This is probably what the period number refers to. The table is useful because all the elements in a particular column have similar chemical properties. It helps to predict how one element will react with another. The atomic number is analogous to the notes on the piano.