Variation of Electronic Configuration along the Periods

Every period starts with an element of general outer configuration $ns^1$ and ends with one of configuration $ns^2\,np^6$ (Section 3.2.1), and the length of each period is fixed by exactly which orbitals get filled, in what order, following the Aufbau principle (Table 3.10).

• Period 1 fills only the 1s orbital, which holds at most 2 electrons -- so period 1 has just 2 elements, hydrogen (1s1) and helium (1s2).
• Period 2 fills 2s then the three 2p orbitals -- 2 + 6 = 8 elements, from lithium (2s1) to neon (2s2 2p6).
• Period 3 fills 3s then 3p the same way -- another 8 elements, sodium to argon.
• Period 4 fills 4s, then (by the Aufbau principle) the 3d orbitals, then 4p -- 2 + 10 + 6 = 18 elements, potassium to krypton.
• Period 5 repeats the same pattern one level up: 5s, 4d, 5p -- 18 elements, rubidium to xenon.
• Period 6 fills 6s, then the 4f orbitals, then 5d, then 6p -- 2 + 14 + 10 + 6 = 32 elements, caesium to radon.
• Period 7 fills 7s, 5f, 6d, 7p in the same way -- another 32 elements, francium to oganesson.

Transition series. In period 4, the filling of the 3d orbitals begins at scandium and ends at zinc; these 10 elements are called the first transition series. The 4d, 5d and 6d orbitals are filled in the same way across the later periods, giving the second, third and fourth transition series respectively. …