The elements Li, C, and F, are all in the period (row) # 2 of the periodic table, and, in consequence, those atoms have two energy levels (main quantum number), n = 1 and n = 2, around the nucleus.The period number of an element in the periodic table is always equal to the number of main energy levels that the atoms have.Li (lithium), C (carbon), and F (fluor) form part of the period 2 of the periodic table.The complete list of elements of the period 2 of the periodic table is Li, Be, B, C, N, O, F, and Ne.The periods are numbered from 1 through 7.The elements of period 1 (H and He) have one main energy level, the elements in period 2 have two main energy levels, the elements in period 3 (Na, Mg, Al, Si, P, O, Cl, and Ne) have three main energy levels. So, as you can infere, the period number always equals the number of main energy levels of the atoms of the element.
