MCAT Chemical and Physical Foundations of Biological Systems Practice Exam 2025 - Free MCAT Practice Questions and Study Guide

The d subshell can hold a maximum of 10 electrons. This is determined by its quantum mechanical description. The d subshell consists of five orbitals, and each orbital can hold a maximum of two electrons with opposite spins, according to the Pauli exclusion principle.

To break it down further, the d subshell contains five distinct orbitals: dxy, dyz, dzx, dx²–y², and dz². Each of these orbitals can accommodate two electrons. Therefore, when you multiply the number of orbitals (5) by the number of electrons each can hold (2), you arrive at the total capacity of the d subshell: 5 orbitals × 2 electrons/orbital = 10 electrons.

This understanding of subshells and their capacity is crucial for grasping the broader concepts of electron configuration and chemical behavior of elements, especially transition metals, which prominently feature d electrons in their electronic structure.