Atomic structure of small- and large-period elements

Large periods consist of two rows (even and odd). Period 4 starts with potassium (K): K 2, 8, 8, 1 — 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹; in Ca the 4s sublevel is filled (4s²). From scandium (Z = 21) to zinc (Z = 30) the electrons fill the inner 3d sublevel — these are d-elements (10 of them): Sc ...3d¹ 4s², Zn ...3d¹⁰ 4s². From gallium to krypton (Z = 31–36) the 4p sublevel fills, and in Kr the outer shell ends with 8 electrons. So period 4 has 2 s-, 10 d- and 6 p-elements, 18 in all. In periods 6 and 7 the 4f and 5f sublevels fill: the 14 elements after lanthanum and actinium are the lanthanides and actinides (f-elements). A formula can be shortened: potassium [Ar] 4s¹, caesium [Xe] 6s¹. A few elements such as chromium and copper have one electron in the outer shell (4s¹); these deviations are studied later.