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Atomic Structure
Bohr's model, quantum numbers, orbitals, electronic configuration, and periodicity.
Class 11Class 12
What You Will Learn in Atomic Structure
Atomic structure describes how protons, neutrons (in the nucleus), and electrons (in shells/orbitals) are arranged within an atom.
- Bohr's model: electrons revolve in fixed circular orbits (shells); energy quantised: En = -13.6/n² eV.
- Four quantum numbers: Principal (n, shell), Azimuthal (l, subshell/shape), Magnetic (ml, orientation), Spin (ms, ±½).
- Aufbau principle: electrons fill orbitals from lowest to highest energy (1s→2s→2p→3s...).
- Hund's rule: orbitals of equal energy (degenerate) are half-filled before any is fully filled.
- Pauli exclusion: no two electrons in an atom can have all four quantum numbers identical.
- Electronic configuration: shorthand notation (e.g., Na = 1s²2s²2p⁶3s¹ or [Ne]3s¹).
Key Formula
Bohr radius: rn = n² × 0.529 Å. Energy: En = -13.6/n² eV. Wavelength (Rydberg): 1/λ = RH(1/n₁² - 1/n₂²), RH = 1.097×10⁷ m⁻¹
Example
The 4th Bohr orbit of hydrogen has radius r₄ = 16 × 0.529 = 8.464 Å.
Solved Numerical Example
Find the wavelength of radiation emitted when an electron in hydrogen jumps from n=4 to n=2. Solution: 1/λ = RH(1/n₁² - 1/n₂²) = 1.097×10⁷(1/4 - 1/16) = 1.097×10⁷ × 3/16 = 2.057×10⁶ m⁻¹. λ = 1/2.057×10⁶ = 486 nm (Balmer series, blue-green visible light).
Expected Exam Questions — Atomic Structure
Q1.State Aufbau principle, Hund's rule, and Pauli exclusion principle.
Answer: Aufbau: electrons fill orbitals in order of increasing energy (n+l rule). Hund's rule: in degenerate orbitals, electrons fill singly with parallel spins before pairing. Pauli Exclusion: no two electrons in an atom can have the same set of all four quantum numbers — max 2 electrons per orbital with opposite spins.
Q2.What is the difference between orbit and orbital?
Answer: Orbit (Bohr model): a fixed, circular path at definite distance from nucleus, 2D concept. Orbital (quantum mechanics): a 3D region of space where the probability of finding an electron is maximum (>90%). Orbitals have shapes: s (spherical), p (dumbbell), d (double dumbbell).
Q3.Calculate the energy of an electron in the 3rd orbit of hydrogen.
Answer: En = -13.6/n² eV. For n=3: E₃ = -13.6/9 = -1.511 eV. Negative sign indicates bound state.
Q4.What are the four quantum numbers? State their values for the outermost electron of Sodium.
Answer: n (principal), l (azimuthal), ml (magnetic), ms (spin). For Na (1s²2s²2p⁶3s¹), outermost electron (3s): n=3, l=0, ml=0, ms=+½.
🔘 MCQ Practice — Atomic Structure
MCQ 1.The maximum number of electrons in the 3rd shell (n=3) is:
A. 8
B. 18 ✓
C. 32
D. 10
✓ Correct Answer: 18
MCQ 2.Which set of quantum numbers is NOT possible?
A. n=2, l=1, ml=0
B. n=3, l=2, ml=-2
C. n=2, l=2, ml=0 ✓
D. n=1, l=0, ml=0
✓ Correct Answer: n=2, l=2, ml=0
📥 Topic Materials
1 resource curated by SII educators for Atomic Structure
🧭 Learning Roadmap
Follow this step-by-step path to master Atomic Structure
1
🌱 Step 1 — Basics
Foundational concepts and definitions