Atomic Structure

Why This Chapter Matters

Atomic structure is foundational JEE Chemistry — 4-8 marks. Quantum numbers, electronic configurations, and Aufbau/Hund's rules are tested in multiple-choice and fill-in questions every year.

Core Concepts

1. Evolution of Atomic Models

Thomson: plum pudding (electrons embedded in positive charge)

Rutherford: nuclear model — small dense positive nucleus, electrons orbiting

Bohr: quantised orbits (see Physics Modern chapter)

Quantum mechanical model: orbitals (probability distributions), not fixed orbits

2. Quantum Numbers (4 numbers define each electron)

Principal (n): Shell number. n = 1,2,3,4... Energy and size.

Azimuthal (l): Subshell. l = 0 to (n-1). Shape.

l=0 (s), l=1 (p), l=2 (d), l=3 (f)

Magnetic (m_l): Orientation. m_l = -l to +l (2l+1 values)

Spin (m_s): +1/2 or -1/2

Max electrons in shell n: 2n²

Max electrons in subshell l: 2(2l+1)

3. Aufbau Principle

Fill orbitals in order of increasing energy:

1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s < 4f < 5d < 6p

Memory aid: use diagonal rule on periodic table subshells

4. Hund's Rule

In degenerate orbitals (same energy, e.g., three 2p orbitals):

Fill one electron each before pairing. All single electrons have same spin (parallel).

Example: Carbon (6e): 1s² 2s² 2p¹_x 2p¹_y 2p⁰_z (not 2p²_x)

5. Pauli Exclusion Principle

No two electrons in an atom can have all four quantum numbers identical.

Max 2 electrons per orbital (opposite spins).

6. Electronic Configuration

Na (11): 1s² 2s² 2p⁶ 3s¹ OR [Ne] 3s¹

Fe (26): [Ar] 3d⁶ 4s²

Cu (29): [Ar] 3d¹⁰ 4s¹ (exception — 3d¹⁰ fully filled is stable)

Cr (24): [Ar] 3d⁵ 4s¹ (exception — 3d⁵ half-filled is stable)

7. Shapes of Orbitals

s: spherical (1 orbital per subshell)

p: dumbbell/figure-8 along axes (3 orbitals: p_x, p_y, p_z)

d: clover-leaf shapes (5 orbitals)

f: complex shapes (7 orbitals)

PYQs

2024: Which quantum numbers are NOT possible: n=2, l=2, m_l=0?

l ranges from 0 to n-1=1. l=2 is NOT possible for n=2. Invalid set.

2023: Write electronic configuration of Cr(24) and explain.

[Ar] 3d⁵ 4s¹. Half-filled d subshell (3d⁵) is extra stable → electron from 4s moves to 3d.

2022: Maximum electrons that can have quantum numbers n=4, l=2?

n=4, l=2 means 4d subshell. 2l+1=5 orbitals. Max 2×5=10 electrons.

Revision Notes

QUANTUM NUMBERS: n, l, m_l, m_s
n = 1,2,3... | l = 0 to n-1 | m_l = -l to +l | m_s = ±1/2

SUBSHELLS: l=0(s), l=1(p), l=2(d), l=3(f)
Electrons: s=2, p=6, d=10, f=14

FILLING ORDER (Aufbau): 1s,2s,2p,3s,3p,4s,3d,4p,5s,4d,...
HUND'S RULE: half-fill before pairing (same spin)
PAULI: max 2e per orbital (opposite spins)

EXCEPTIONS:
Cu: [Ar]3d¹⁰4s¹ (full d preferred over 3d⁹4s²)
Cr: [Ar]3d⁵4s¹ (half-full d preferred over 3d⁴4s²)