ChemistryCHEM 01
Atomic Structure
Bohr model, quantum numbers, atomic orbitals, electronic configuration, periodic properties
Concept Core
Essential theory — everything NCERT tests on Atomic Structure
BOHR'S MODEL
Electrons in fixed circular orbits. Energy: En = −13.6/n² eV (H atom). Radius: rn = 0.529n² Å. Energy absorbed = E₂−E₁ (excitation). Emitted = E₁−E₂ (de-excitation).
Spectral series: Lyman (UV, n→1), Balmer (visible, n→2), Paschen (IR, n→3), Brackett (n→4), Pfund (n→5).
QUANTUM NUMBERS
n (principal): shell, energy, size. n=1,2,3...
l (azimuthal): subshell shape. 0≤l≤n−1. s=0,p=1,d=2,f=3.
ml (magnetic): orbital orientation. −l to +l. Total 2l+1 values.
ms (spin): +½ or −½. Two electrons per orbital.
QUANTUM MECHANICAL MODEL
Heisenberg uncertainty: Δx·Δp ≥ h/4π. Cannot simultaneously know exact position and momentum.
Orbital: region of 90% probability of finding electron. Shapes: s (sphere), p (dumbbell), d (cloverleaf), f (complex).
Nodes: radial = n−l−1. Angular = l. Total = n−1.
ELECTRONIC CONFIGURATION
Aufbau: fill lowest energy first. Energy order: 1s<2s<2p<3s<3p<4s<3d<4p...
Pauli exclusion: no two electrons same set of 4 quantum numbers. Max 2e per orbital.
Hund's rule: maximum spin multiplicity. Fill each orbital singly before pairing.
Exceptions: Cr = [Ar]3d⁵4s¹; Cu = [Ar]3d¹⁰4s¹.
PERIODIC PROPERTIES
Atomic radius: increases down group, decreases across period (left to right).
Ionisation energy: decreases down group, increases across period. Exceptions: IE(Be)>IE(B), IE(N)>IE(O).
Electronegativity: F highest. Increases across period, decreases down group. Electron affinity: Cl highest (not F — small size, repulsion).
Fact & Formula Vault
High-yield facts, numbers, and formulas
Bohr Model
E_n = −13.6/n² eV
r_n = 0.529n² Å
Lyman: n→1 (UV)
Balmer: n→2 (visible)
Quantum Numbers
n: shell (1,2,3...)
l: 0 to n−1 (subshell)
ml: −l to +l (orbital)
ms: +½ or −½ (spin)
Key Rules
Aufbau: lowest E first
Pauli: max 2e per orbital
Hund: max unpaired spins
Cr=[Ar]3d⁵4s¹ (exception)
Worked Examples
NEET-style questions solved step-by-step
EASYThe spectral line in hydrogen spectrum visible to naked eye belongs to which series?▾
The spectral line in hydrogen spectrum visible to naked eye belongs to which series?
Balmer series — transitions to n=2. Lies in visible region (400–700 nm). Lyman = UV. Paschen, Brackett, Pfund = IR.
MEDIUMWhich of the following sets of quantum numbers is NOT possible? n=2, l=2?▾
Which of the following sets of quantum numbers is NOT possible? n=2, l=2?
Not possible: l must be 0 to n−1. For n=2, l can be 0 or 1 only. l=2 is not allowed for n=2.
HARDChromium (Z=24) has electronic configuration [Ar]3d⁵4s¹ instead of [Ar]3d⁴4s². Why?▾
Chromium (Z=24) has electronic configuration [Ar]3d⁵4s¹ instead of [Ar]3d⁴4s². Why?
Half-filled (3d⁵) is extra stable due to symmetry and exchange energy. So one electron from 4s promotes to 3d: [Ar]3d⁵4s¹ is more stable than [Ar]3d⁴4s².
Mistake DNA
Common NEET traps for this chapter
⚠ Balmer vs Lyman
Lyman: n→1 (UV, not visible). Balmer: n→2 (VISIBLE). Most NEET questions ask about the visible series = Balmer.
✓ Fix: Visible = Balmer (n→2). UV = Lyman (n→1).
⚠ Electron affinity maximum
F does NOT have the highest electron affinity — Cl does. F is too small; adding electron causes repulsion.
✓ Fix: Cl > F for electron affinity. F > Cl for electronegativity.
⚠ IE exceptions
IE(Be) > IE(B): 2s is more stable than 2p. IE(N) > IE(O): half-filled 2p³ is extra stable.
✓ Fix: Be>B and N>O for 1st IE — two exceptions across period 2.
Chapter Intelligence
Exam data and last-minute strategy
NEET Frequency
2–3 Q/year. Balmer series, quantum number rules, electronic configuration (Cr/Cu exceptions), IE/EA trends are most tested.
High-Yield
Balmer = visible. E_n = −13.6/n². l = 0 to n−1. Hund's rule. Cr = 3d⁵4s¹. Cu = 3d¹⁰4s¹. IE: Be>B, N>O.
Strategy
Quantum number problems: check all 4 rules simultaneously. Electronic configuration: know exceptions Cr and Cu. Periodic trends: know the two IE exceptions.
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