JEE Main Chemistry Atomic Structure 2027: Quantum Numbers, Orbitals & Bohr Model

Why does chromium break the filling rule while its neighbours obey it? Why does a 3p orbital have exactly one radial node? Atomic Structure answers these with a small set of rules, and JEE Main tests them again and again in short, predictable ways.

Four quantum numbers tell you everything about where an electron lives.

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Chapter at a Glance

Priority reflects past-paper trends, not an official NTA weightage.

What the NTA Syllabus Covers

• Nature of electromagnetic radiation, photoelectric effect, spectrum of the hydrogen atom
• Bohr model: postulates, energy and radii of orbits, limitations
• Dual nature of matter, de Broglie relationship, Heisenberg uncertainty principle
• Quantum mechanical model, orbitals as wave functions, quantum numbers, shapes of s, p and d orbitals
• Aufbau principle, Pauli's exclusion principle, Hund's rule, extra stability of half-filled and fully filled orbitals

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Master These Topics

1. Bohr's Model of Hydrogen-like Species

For a species with atomic number Z:

• E_n = −13.6 Z² / n² eV (or −2.18 × 10⁻¹⁸ Z²/n² J)
• r_n = 52.9 n² / Z pm

Worked example: For the hydrogen transition n = 4 to n = 2, ΔE = 13.6 (1/4 − 1/16) = 2.55 eV. Wavelength = 1240 / 2.55 ≈ 486 nm, a visible Balmer line. Bohr's model fails for multi-electron atoms and cannot explain fine spectral lines or the Zeeman effect.

2. Wave Nature and Uncertainty

de Broglie: λ = h / mv. Worked example: An electron moving at 10⁶ m/s has λ = 6.626 × 10⁻³⁴ / (9.11 × 10⁻³¹ × 10⁶) ≈ 7.3 × 10⁻¹⁰ m, about 0.73 nm, comparable to atomic dimensions. A cricket ball's wavelength is far too small to observe.

Heisenberg uncertainty principle: Δx · Δp ≥ h / 4π. It is significant only for microscopic particles.

3. Quantum Numbers and Orbital Counting

A shell n holds n² orbitals and 2n² electrons. Subshells s, p, d hold 1, 3, 5 orbitals.

Nodes: radial nodes = n − l − 1, angular nodes = l, total nodes = n − 1. For 3p: radial nodes = 3 − 1 − 1 = 1, angular nodes = 1, total 2.

Trap: Do not count the nucleus as a node. And for l = 0 (s orbitals) there are no angular nodes, so all nodes are radial.

4. Filling Rules and Exceptions

• Aufbau: fill in increasing order of (n + l); when equal, lower n first.
• Pauli: no two electrons share the same four quantum numbers.
• Hund: degenerate orbitals fill singly with parallel spins before pairing.

Half-filled and fully filled subshells are extra stable, which explains the two classic exceptions: chromium is [Ar] 3d⁵ 4s¹ (not 3d⁴ 4s²) and copper is [Ar] 3d¹⁰ 4s¹ (not 3d⁹ 4s²).

For cations of transition metals, the 4s electrons are removed before the 3d electrons.

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Common Traps to Avoid

• Forgetting that for a given n, l can only go up to n − 1.
• Writing Cr and Cu configurations with the regular filling order.
• Removing 3d electrons before 4s when forming transition metal cations.
• Using the Bohr formula for multi-electron atoms.

60-Second Revision Sheet

• E_n = −13.6 Z²/n² eV, r_n = 52.9 n²/Z pm
• Radial nodes n − l − 1; angular nodes l; total n − 1
• λ = h/mv; Δx · Δp ≥ h/4π
• Exceptions: Cr 3d⁵4s¹, Cu 3d¹⁰4s¹

Your Study Plan

1. Day 1: Bohr model and hydrogen spectrum numericals.
2. Day 2: quantum numbers, allowed sets and node counting.
3. Day 3: electronic configuration including ions and exceptions.
4. Day 4: de Broglie, uncertainty and a timed mixed set.

Practice Atomic Structure Questions Free → (opens in a new tab)

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Continue Your Chemistry Journey

• Previous chapter: Basic Concepts in Chemistry
• Next chapter: Chemical Bonding & Molecular Structure
• All JEE Main Chemistry chapters
• Complete JEE Main Syllabus 2027 guide

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