The first ionization enthalpy of aluminium is lower than that of magnesium. — Periodic Table and Periodicity Chemistry Question

💡 Solution & Explanation

Step 1: Identify which subshells are being filled during the fourth period: the 4s, 3d, and 4p subshells. Step 2: Note that the 4d subshell only begins filling in the fifth period (starting with Yttrium, Z=39). Step 3: Since the filling of 4d orbitals does not occur in any fourth-period elements in their ground state, the number of elements with 4d electrons is 0.