How many total electrons can the d subshell hold?

The d subshell can hold a total of 10 electrons. The reasoning behind this lies in the structure of the subshell itself. The d subshell consists of five d orbitals (dxy, dyz, dxz, dx²-y², dz²). Each orbital can accommodate a maximum of two electrons, following the Pauli Exclusion Principle, which states that no two electrons in the same atom can have identical sets of quantum numbers.

Thus, with five orbitals, each holding two electrons, the total calculation is 5 orbitals multiplied by 2 electrons per orbital, resulting in 10 electrons. This is a fundamental concept in understanding electron configurations, especially when dealing with transition metals where d block elements are involved.