Electronic configuration, periods and groups

Where an element sits in the table reflects the quantum numbers of the last orbital to be filled.

The period number is n of the outermost (valence) shell. Each period holds twice as many elements as there are orbitals being filled in it.

Period 1 (1s): H and He, 2 elements. Period 2 fills 2s and 2p, Li to Ne, 8 elements. Period 3 fills 3s and 3p, Na to Ar, 8 elements.

Period 4 begins at K (4s). Before 4p, the 3d orbitals fill: the 3d transition series runs from Sc (Z = 21, 3d¹4s²) to Zn (Z = 30, 3d¹⁰4s²). The period ends at Kr after 4p, 18 elements in all.

Period 5 begins at Rb, contains the 4d series starting at Y (Z = 39) and ends at Xe after 5p: 18 elements, because 5s, 4d and 5p offer 9 orbitals, room for 18 electrons.

Period 6 has 32 elements, filling 6s, 4f, 5d and 6p. The 4f series (lanthanoids) runs from Ce (Z = 58) to Lu (Z = 71).

Period 7 fills 7s, 5f, 6d and 7p and holds most of the man-made radioactive elements; it closes with element 118, a noble gas. Filling 5f after Ac (Z = 89) gives the actinoid series.

Elements in one group have the same number and arrangement of outer electrons, hence similar properties. Group 1 all end in ns¹: Li [He]2s¹, Na [Ne]3s¹, K [Ar]4s¹, Rb [Kr]5s¹, Cs [Xe]6s¹, Fr [Rn]7s¹.

Conclusion: properties depend periodically on atomic number, not on relative atomic mass.