Electronic Configuration + Quantum Numbers
Atomic Structure
42
JEE Qs
5%
Hard
75
min
Mastering electronic configuration and quantum numbers is fundamental, as it underpins understanding of periodicity, chemical bonding, and properties of elements.
🧮 Key Formulas
✅ Key Points for JEE
- 1Understand the physical significance of each quantum number (n, l, m_l, m_s) in defining the electron's state and orbital properties.
- 2Master the systematic application of Aufbau principle, Hund's rule, and Pauli's exclusion principle to correctly write electronic configurations for elements and ions.
- 3Identify and explain common exceptions to the Aufbau principle (e.g., Cr, Cu, Mo, Ag, Au) based on enhanced stability of half-filled or completely filled subshells.
- 4Be able to deduce all four quantum numbers for any specific electron in an atom given its electronic configuration.
- 5Recognize valid and invalid sets of quantum numbers based on their interdependencies (e.g., l must be < n, |m_l| must be <= l).
⚠️ Common Mistakes
- ✕Incorrectly applying Hund's rule by pairing electrons in degenerate orbitals before all orbitals are singly occupied.
- ✕Confusing the order of filling orbitals (Aufbau principle) with the order of electron removal for transition metal cations (electrons are removed from outermost shell first, e.g., 4s before 3d).
- ✕Failing to recall or apply exceptions to Aufbau principle for elements like Cr, Cu, etc., leading to incorrect configurations.
- ✕Assigning impermissible quantum numbers, such as l values greater than or equal to n, or m_l values outside the range of -l to +l.
- ✕Incorrectly determining the (n+l) value for orbital energy comparison, especially for higher principal quantum numbers.
📝 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