Aufbau Principle, Pauli's Exclusion Principle and Hund's Rule — Prelims Strategy

Prelims Strategy

For NEET Prelims, a solid strategy for this topic involves three key areas: conceptual clarity, application of rules, and memorization of exceptions. Firstly, ensure you clearly understand what each principle states and why it's important.

Don't just memorize definitions; internalize the 'why' behind each rule (e.g., why parallel spins in Hund's rule, why opposite spins in Pauli's). Secondly, practice applying the Aufbau principle using the $(n+l)$ rule extensively.

Be quick and accurate in determining the filling order. For numerical problems, such as finding the number of electrons in a subshell, remember the formulas for the number of orbitals ($2l+1$) and apply Pauli's principle (2 electrons per orbital).

Thirdly, commit the exceptions to the Aufbau principle (especially Cr and Cu) to memory, along with their explanations (half-filled/fully-filled stability). When solving MCQs, carefully read the question to identify which rule is being tested.

For electron configuration questions, systematically fill electrons, checking for violations of each rule. For 'NOT allowed' questions, meticulously check each quantum number against its permissible range.

Trap options often involve subtle violations of Hund's rule or incorrect $(n+l)$ order. Visualize orbital diagrams (box notation) to quickly identify unpaired electrons or rule violations.