Electronic Configuration — Aufbau, Pauli, Hund's rule
Atomic Structure
9
JEE Qs
8%
Hard
60
min
Master the orbital filling order, practice exceptions rigorously, and understand electron removal for ions to avoid common pitfalls.
🧮 Key Formulas
✅ Key Points for JEE
- 1Aufbau Principle states that electrons occupy the lowest energy orbitals available first. The sequential filling order is crucial.
- 2Pauli Exclusion Principle dictates that an orbital can hold a maximum of two electrons, and these two electrons must have opposite spins (one +1/2, one -1/2).
- 3Hund's Rule of Maximum Multiplicity states that for degenerate orbitals (same energy, e.g., p-orbitals, d-orbitals), electrons will first occupy each orbital singly with parallel spins before any pairing occurs.
- 4Half-filled (e.g., p³, d⁵, f⁷) and completely filled (e.g., p⁶, d¹⁰, f¹⁴) subshells exhibit extra stability due to symmetry and exchange energy, leading to exceptions in the Aufbau principle (e.g., Cr, Cu, Mo, Ag, Au).
- 5When forming ions, electrons are first removed from the outermost shell (largest 'n' value) before considering 'd' orbitals, even if 'd' orbitals were filled last in the neutral atom (e.g., for transition metals, 4s electrons are lost before 3d electrons).
⚠️ Common Mistakes
- ✕Incorrectly recalling the energy order of orbitals, especially for 4s vs 3d, leading to errors in filling.
- ✕Failing to apply Hund's rule correctly, resulting in premature pairing of electrons in degenerate orbitals.
- ✕Forgetting common exceptions to the Aufbau principle (Cr, Cu, Mo, Ag, Au) due to half-filled/fully-filled subshell stability.
- ✕Incorrectly determining electron configurations for ions, especially transition metals, by removing electrons from the last filled orbital instead of the outermost shell.
📝 Practice Questions
See allQ57.Given below are two statements : Statement (I) : For a given shell, the total number of allowed orbitals is given by n2 . Statement (II) : For any subshell, the spatial orientation of the orbitals is given by −l to +l values including zero. In the light of the above statements, choose the correct answer from the options given below : (1) Both Statement I and Statement II are false (2) Statement I is true but Statement II is false (3) Both Statement I and Statement II are true (4) Statement I is false but Statement II is true
Q57.In a multielectron atom, which of the following orbitals described by three quantum numbers will have same energy in absence of electric and magnetic fields? A. n = 1, l = 0, m1 = 0 B. n = 2, l = 0, m1 = 0 C. n = 2, l = 1, m1 = 1 D. n = 3, l = 2, m1 = 1 E. n = 3, l = 2, m1 = 0 Choose the correct answer from the options given below: (1) B and C Only (2) A and B Only (3) C and D Only (4) D and E Only
Q51.Radius of the first excited state of Helium ion is given as : a0 → radius of first stationary state of hydrogen atom. (1) r = 4a0 (2) r = 2a0 (3) r = a02 (4) r = a04
Q62.Given below are two statements : Statement (I) : A spectral line will be observed for a 2px →2py transition. Statement (II) : 2Px and 2py are degenerate orbitals. In the light of the above statements, choose the correct answer from the options given below : (1) Both Statement I and Statement II are true (2) Statement I is false but Statement II is true (3) Both Statement I and Statement II are false (4) Statement I is true but Statement II is false
Q51.For hydrogen atom, the orbital/s with lowest energy is/are : (A) 4 s (B) 3px (C) 3 dx2−y2 (D) 3 dz2 (E) 4pz Choose the correct answer from the options given below : (1) (B), (C) and (D) only (2) (A) and (E) only (3) (A) only (4) (B) only
Q44.The frequency of revolution of the electron in Bohr's orbit varies with n, the principal quantum number as (1) 1 (2) 1 n4 n2 (3) 1 (4) 1 n n3
NCERT Chapters
- Class 11 Chemistry Ch 2: Structure of Atom