Based on the Aufbau principle and general electronic configurations, exactly how many elements prese — Periodic Table and Periodicity Chemistry Question

💡 Solution & Explanation

The 3rd period corresponds to the principal quantum number $n=3$. The sequence of subshell filling for this period involves only $3s$ and $3p$. According to the $(n+l)$ rule, the $3d$ subshell ($n+l = 3+2=5$) is of higher energy than the $4s$ subshell ($n+l = 4+0=4$) and begins filling only in the 4th period. Therefore, zero elements in the 3rd period have 3d electrons.