According to the Aufbau principle and the structure of the long form of the periodic table, how many — Periodic Table and Periodicity Chemistry Question

💡 Solution & Explanation

The 4th period corresponds to the principal quantum number $n=4$. The subshells filled are $4s, 3d,$ and $4p$. The maximum capacity of these subshells is $2 + 10 + 6 = 18$ electrons. Therefore, the 4th period contains 18 elements.