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Atomic Structure

Bohr model, quantum numbers, electronic configuration

Key Concepts
  • Bohr model: electrons in circular orbits; quantised energy and angular momentum
  • Quantum numbers: n (principal), l (azimuthal), m_l (magnetic), m_s (spin)
  • Aufbau principle: fill lowest energy orbitals first
  • Hund's rule: maximum unpaired electrons in degenerate orbitals
  • Pauli exclusion principle: no two electrons can have identical quantum numbers
Important Formulae
Bohr energy (hydrogen) E_n = −13.6/n² eV
de Broglie wavelength λ = h/mv
Heisenberg uncertainty Δx · Δp ≥ h/4π
Orbital angular momentum L = √[l(l+1)] · h/2π
Max electrons in shell n 2n²
Quick Tips
  • l ranges from 0 to n−1; m_l ranges from −l to +l; m_s = +½ or −½.
  • Order of filling: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p … (n+l rule).
  • Cr is [Ar]3d⁵4s¹ and Cu is [Ar]3d¹⁰4s¹ — exceptions due to extra stability of half-filled/fully-filled d-orbitals.
Sample Practice Questions

  1. Isotopes have the same:

    • Mass number only
    • Both atomic and mass number
    • Atomic number but different mass number
    • Number of neutrons only
    Show answer

    Answer: Atomic number but different mass number

  2. The energy of a photon E = hν. At wavelength 400 nm (h = 6.63×10⁻³⁴, c = 3×10⁸):

    • ~2.5 × 10⁻¹⁹ J
    • ~4.97 × 10⁻¹⁹ J
    • ~1.0 × 10⁻¹⁸ J
    • ~6.63 × 10⁻³⁴ J
    Show answer

    Answer: ~4.97 × 10⁻¹⁹ J

  3. Hund's rule states that electrons occupy degenerate orbitals:

    • In pairs first
    • Singly before pairing
    • In any random order
    • In the lowest energy orbital only
    Show answer

    Answer: Singly before pairing

  4. The 3d subshell can hold maximum of:

    • 2 electrons
    • 6 electrons
    • 10 electrons
    • 14 electrons
    Show answer

    Answer: 10 electrons

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Practice Questions

Practice 20 randomly selected NEET questions on Atomic Structure. Answers are revealed after each question.

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