Questions
4–6 questions in board and competitive papers
Difficulty
Medium
Importance
Foundation core — never skip
Overview
The Structure of Atom is the foundational chapter of Chemistry that explains the subatomic nature of matter. It is critical for competitive exams as it forms the basis for understanding periodic trends, chemical bonding, and quantum mechanics.
Bohr Model of Atom
Bohr model addressed the stability of atoms by postulating fixed circular orbits with quantized energy levels. It is the bridge between classical physics and the modern quantum mechanical view.
- Energy is quantized: E = -13.6/n^2 eV
- Angular momentum is quantized: mvr = nh/2π
- Applicable only to hydrogen-like species (single electron)
- Emission spectrum occurs during transition from higher to lower energy levels
Quantum Numbers
Quantum numbers are the 'address' of an electron within an atom, defined by wave functions. Mastering these is essential for identifying electronic properties and orbital shapes.
- Principal quantum number (n): shell or size
- Azimuthal quantum number (l): subshell or shape (0 to n-1)
- Magnetic quantum number (m_l): orientation (-l to +l)
- Spin quantum number (m_s): electron spin (+1/2 or -1/2)
Electronic Configuration
The distribution of electrons into various shells and orbitals follows specific energetic principles. Questions frequently test exceptions to these standard rules.
- Aufbau Principle: Filling orbitals from lowest to highest energy
- Pauli Exclusion Principle: No two electrons have same set of 4 quantum numbers
- Hund's Rule: Maximum multiplicity in degenerate orbitals
- Stability of half-filled and fully-filled orbitals (e.g., Cr: 3d5 4s1)
Formula Sheet
E = -13.6(Z^2/n^2) eV
mvr = nh/2π
Energy difference ΔE = hν = R_H * (1/n1^2 - 1/n2^2)
n + l rule for orbital energy
Exam Tip
Always verify the n+l rule for orbital energy sequence before writing down electronic configurations to avoid errors in high-Z elements.
Common Mistakes
- Applying the Bohr formula for multi-electron atoms like Helium or Lithium incorrectly.
- Ignoring the stability of half-filled/fully-filled d-orbitals when writing configurations for Transition metals.
- Confusing the radial nodes formula (n - l - 1) with angular nodes formula (l).
More Revision Notes
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