La (lanthanum) having atomic number 57 is a member of : — Periodic Table and Periodicity Chemistry Question

💡 Solution & Explanation

Step 1: The d-orbitals only exist starting from the principal quantum number n=3, which means the (n-1)d subshell is first filled when n=4. Step 2: Substitute n=4 into the configuration (n-1)d^6 ns^2 to obtain 3d^6 4s^2. Step 3: This electronic configuration belongs to Iron (Fe, Z=26), which is in the d-block of the fourth period. Thus, the element is located in the fourth period.