Answer :
Final answer:
Option b, in the periodic table, the 4s subshell fills before the 3d subshell, which is why transition metals have electrons added to the 3d subshell after the 4s subshell is filled.
Explanation:
When going down the periodic table, the 4s subshell is filled before the 3d subshell. This sequence is reflected in the periodic table's structure, where after the 4s subshell is completely filled with two electrons, the 3d subshell is then filled with up to 10 electrons. This pattern contributes to the distinct section of 10 transition metals in the middle of the periodic table. The exceptions to this pattern arise due to certain subshell electron clouds being closer to the nucleus and therefore lower in energy, making the s subshells fill before the d subshells at higher principal quantum numbers.
