What is the minimum number of electrons required to fill the M shell completely?
The M shell (n = 3) has a maximum capacity of 2n² = 2 × 3² = 18 electrons, assuming all subshells (3s, 3p, 3d) are filled. This follows from NCERT principle where the relation explains the outcome clearly for students in simple steps.
Ref: NCERT Chemistry Textbook for Class XI and XII, Chapter: Relevant Chemistry topic covering principles and examples as per NCERT.