JEE Main Atomic Structure 2027 — Bohr Model, Quantum Numbers, Spectra and 35 Practice MCQs

Last Updated: May 2026

JEE Main Atomic Structure 2027 is NCERT Class XI Chapter 2 — the foundational chapter that powers every later quantum/bonding/periodicity problem. In JEE Main 2024-25 across 16 shifts, Atomic Structure contributed an average of 1.6 questions / 6.4 marks per shift, with the Bohr model + spectrum + quantum numbers triad accounting for 70% of those items. NEET-aligned reasoning makes overlap with Inorganic 1 strong, so this chapter is also a “build a complete answer for one MCQ but lock 4 chapters of trivia” deal.

This guide covers the entire JEE Main Atomic Structure syllabus — Bohr model, hydrogen spectrum, de Broglie wavelength, Heisenberg uncertainty, quantum numbers, Aufbau / Hund / Pauli, photoelectric effect, electronic configurations — followed by 35 JEE-style practice MCQs with full answer key, an interactive 10-MCQ quiz, and a JEE 2027 FAQ.

1. Sub-atomic Particles — Quick Recap

2. Atomic Models — Quick Hierarchy

• Dalton (1808) — atom indivisible.
• Thomson (1898) — plum pudding (electrons embedded in positive sphere).
• Rutherford (1911) — gold-foil α-scattering → nuclear model. Failure: predicted unstable atom (electron should spiral into nucleus by Maxwell’s equations).
• Bohr (1913) — quantised circular orbits; resolved hydrogen spectrum.
• Sommerfeld (1916) — elliptical orbits; introduced ℓ.
• Quantum mechanical (Schrödinger 1926) — orbitals, ψ², four quantum numbers.

3. Bohr Model of the Hydrogen Atom (HEAVY JEE)

Bohr’s three postulates:

1. Electrons revolve in stationary orbits without emitting energy.
2. Angular momentum is quantised: mvr = nh/(2π).
3. Transition between orbits emits/absorbs energy: hν = E2 − E1.

Bohr Formulas (memorise — pure plug-and-play marks)

Key inferences (a JEE staple):

• Going from n=1 to n=2 in H atom: r×4, v÷2, E×(1/4) (becomes less negative).
• For He+ (Z=2, n=1): r=0.529/2 = 0.2645 Å, E=−54.4 eV.
• Ionisation energy of H = 13.6 eV; Li²⁺ = 13.6×9 = 122.4 eV.

4. Hydrogen Atom Spectrum (Series)

Rydberg formula: 1/λ = R_H·Z²·(1/n₁² − 1/n₂²), where R_H = 1.097×10⁷ m⁻¹.

Number of spectral lines when an electron drops from n=n to n=1: n(n−1)/2.

5. Limitations of Bohr Model

• Fails for multi-electron atoms.
• Cannot explain fine structure of spectral lines (resolved by Sommerfeld).
• Cannot explain Zeeman / Stark effects (splitting in magnetic / electric fields).
• Violates Heisenberg uncertainty principle (precise orbit + momentum is forbidden).

6. Wave-Particle Duality — de Broglie

λ = h / p = h / (mv). Significant only for sub-atomic particles. For an electron at 1 V acceleration: λ = 12.27/√V Å.

7. Heisenberg Uncertainty Principle

Δx · Δp ≥ h/(4π) — also expressed as Δx · Δv ≥ h/(4πm). Direct corollary: an electron in an atom cannot have a precisely defined orbit (wipes out Bohr’s classical picture).

8. Quantum Numbers — the Address of an Electron

ℓ = 0 → s (1 orbital), ℓ = 1 → p (3 orbitals), ℓ = 2 → d (5 orbitals), ℓ = 3 → f (7 orbitals).

Total electrons in shell n: 2n². In a subshell ℓ: 2(2ℓ+1).

Nodes

• Total nodes = n − 1.
• Angular nodes = ℓ.
• Radial nodes = n − ℓ − 1.

9. Rules of Filling — Electronic Configuration

1. Aufbau principle: lower (n+ℓ) fills first; if (n+ℓ) ties, lower n fills first. Order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p.
2. Pauli exclusion: no two electrons in an atom can have all four quantum numbers identical. Each orbital holds ≤2 electrons of opposite spin.
3. Hund’s rule of maximum multiplicity: when degenerate orbitals are available, electrons occupy each singly with parallel spins before pairing.

Anomalous Configurations (JEE staples)

10. Photoelectric Effect (Einstein 1905)

hν = W₀ + ½mv², where W₀ = work function = hν₀ (threshold frequency).

• If ν < ν₀: no emission, regardless of intensity.
• If ν > ν₀: kinetic energy depends on ν − ν₀, NOT on intensity.
• Number of photoelectrons ∝ intensity.

11. Schrödinger Equation & Orbital Concept

ψ is the wavefunction; ψ² is the probability density of finding an electron. An orbital is the 3D region where probability of finding an electron is high (typically >90%). Crucially: orbitals replace orbits.

Useful internal links: JEE 2027 Drishti Chemistry · JEE 2027 syllabus · free chapter notes · JEE 2027 FAQ · join Drishti JEE Chemistry batch.

35 JEE Main Practice MCQs — Atomic Structure

1. Energy of electron in nth orbit of H atom: (a) −13.6/n² eV (b) +13.6 eV (c) −13.6n² eV (d) 13.6/n eV
2. Bohr’s radius for H atom is: (a) 0.529 nm (b) 0.529 Å (c) 5.29 Å (d) 1 Å
3. Velocity of electron in 1st orbit of He⁺ is: (a) 2.18×10⁶ m/s (b) 4.36×10⁶ m/s (c) 1.09×10⁶ m/s (d) 5.45×10⁶ m/s
4. Lyman series falls in: (a) UV (b) Visible (c) IR (d) Microwave
5. Balmer series corresponds to n₁ = (a) 1 (b) 2 (c) 3 (d) 4
6. Number of spectral lines from n=4 to n=1: (a) 3 (b) 4 (c) 6 (d) 10
7. Ionisation energy of Li²⁺ is: (a) 13.6 eV (b) 27.2 eV (c) 122.4 eV (d) 54.4 eV
8. Quantisation of angular momentum: mvr = (a) nh (b) nh/π (c) nh/(2π) (d) nh/(4π)
9. de Broglie wavelength of electron at 100 V: (a) 12.27 Å (b) 1.227 Å (c) 0.1227 Å (d) 122.7 Å
10. Heisenberg’s uncertainty: Δx·Δp ≥ (a) h (b) h/2 (c) h/(4π) (d) h/π
11. For an electron, ℓ = 2 means: (a) s (b) p (c) d (d) f
12. Number of orbitals in d subshell: (a) 1 (b) 3 (c) 5 (d) 7
13. Maximum electrons in M shell: (a) 2 (b) 8 (c) 18 (d) 32
14. For 3p orbital, n+ℓ = (a) 3 (b) 4 (c) 5 (d) 6
15. Number of radial nodes in 3p: (a) 0 (b) 1 (c) 2 (d) 3
16. Total nodes in 4s orbital: (a) 1 (b) 2 (c) 3 (d) 4
17. Cr (Z=24) configuration: (a) [Ar] 3d⁴ 4s² (b) [Ar] 3d⁵ 4s¹ (c) [Ar] 3d⁶ 4s⁰ (d) [Ar] 4s² 3d⁴
18. Cu (Z=29) configuration: (a) [Ar] 3d⁹ 4s² (b) [Ar] 3d¹⁰ 4s¹ (c) [Ar] 3d⁸ 4s³ (d) [Ar] 4s¹ 3d¹⁰ — same as b
19. Pauli exclusion forbids: (a) two electrons in same orbital (b) two electrons with same spin in same orbital (c) two electrons with all four QN identical (d) odd electrons
20. Hund’s rule applies to: (a) Filling within a shell (b) Filling of degenerate orbitals (c) Pauli pairing (d) Aufbau order
21. The set of QN (3,2,−2,+½) corresponds to which orbital: (a) 3p (b) 3d (c) 3s (d) 4d
22. Photoelectric current depends on: (a) Frequency only (b) Intensity (c) Work function only (d) Wavelength only
23. KE of photoelectron depends on: (a) Frequency above threshold (b) Intensity (c) Work function only (d) Number of photons
24. Threshold frequency relation: (a) hν₀ = W₀ (b) ν₀ = h/W₀ (c) hν₀ = mv (d) hν₀ = ½mv²
25. Mass of photon at rest is: (a) 9.1×10⁻³¹ (b) 0 (c) 1.67×10⁻²⁷ (d) Planck’s constant
26. Wavefunction ψ has physical meaning: (a) ψ itself = probability (b) ψ² = probability density (c) ψ³ = energy (d) ψ⁻¹ = orbital size
27. Number of unpaired electrons in Fe³⁺ (Z=26): (a) 4 (b) 5 (c) 3 (d) 6
28. (n+ℓ) value for 4f: (a) 5 (b) 6 (c) 7 (d) 8
29. Order of filling using Aufbau: (a) 4s < 3d (b) 3d < 4s (c) Both equal (d) 3d before 3p
30. Wavelength of K-α X-ray varies with Z as: (a) Moseley’s law: √ν ∝ (Z−b) (b) ν ∝ Z (c) ν ∝ Z² (d) ν ∝ 1/Z
31. Total spectral lines from n=5 to ground: (a) 6 (b) 10 (c) 15 (d) 4
32. Energy required to remove electron from n=2 of H: (a) 13.6 eV (b) 3.4 eV (c) 27.2 eV (d) 6.8 eV
33. Speed of e⁻ in 2nd orbit of H is half of: (a) 1st orbit (b) Light (c) Sound (d) Itself
34. Number of orbitals when n=3: (a) 4 (b) 9 (c) 16 (d) 18
35. For an electron in 3d orbital, the magnetic QN cannot be: (a) −2 (b) 0 (c) +3 (d) +1

Answer Key (JEE Atomic Structure Practice)

Interactive 10-MCQ Quiz

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FAQ — JEE Main Atomic Structure 2027

How many JEE Main questions come from Atomic Structure?

Average 1.6 questions / 6.4 marks per shift across 2024-25. The Bohr-energies + spectrum + quantum numbers triad accounts for ~70% of those items.

What is the most common Bohr-model question?

Energy / radius / velocity ratio between two states of a hydrogen-like ion. JEE drills the proportionality: r ∝ n²/Z, v ∝ Z/n, E ∝ −Z²/n². Memorise this row of three.

Why does Cr have 3d⁵ 4s¹ instead of 3d⁴ 4s²?

Half-filled d-subshell (3d⁵) gives extra exchange-energy stability, so an electron from 4s shifts to 3d. Cu (3d¹⁰ 4s¹) follows the same logic but for fully-filled stability.

What is the difference between an orbit and an orbital?

Orbit = fixed circular path (Bohr — disproven by uncertainty principle). Orbital = 3D probability cloud where electron is found ≥90% of the time (quantum mechanical model — Schrödinger). JEE always tests this distinction.

How is the photoelectric effect linked to Bohr’s quantum idea?

Both confirm energy is absorbed/released in discrete packets (quanta of hν). JEE asks for Einstein’s equation hν = W₀ + ½mv² and the cut-off threshold property — KE depends on frequency, not intensity.

Atomic Structure is your foundation for every later inorganic and physical chemistry chapter in JEE. Drishti JEE Chemistry batches at JEE Gurukul include weekly chapter sprints, Bohr-formula speed drills and full-length JEE Main mocks. Explore all JEE 2027 courses →