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Physics · Unit 15

Atomic Spectra: MDCAT Notes and MCQs

Types of spectra

  • Continuous spectrum: all wavelengths (hot solids/liquids, filament lamp).
  • Line (emission) spectrum: discrete bright lines from excited gases/vapours at low pressure — each element has a unique set ("fingerprint") → used to identify elements (spectroscopy).
  • Absorption spectrum: dark lines on a continuous background at the same wavelengths the element emits (e.g. Fraunhofer lines in sunlight).

Explanation — Bohr model

  • Electrons occupy discrete energy levels: En = −13.6/n² eV (hydrogen). Ground state −13.6 eV → ionization energy 13.6 eV.
  • When an electron jumps from a higher level n2 to a lower level n1, it emits a photon: hf = En2 − En1. Discrete energy differences → discrete lines.
  • Radius rn = 0.53 n² Å; angular momentum mvr = nh/2π.
  • Wave-number formula: 1/λ = RH(1/n1² − 1/n2²), RH = 1.097 × 107 m−1.

Hydrogen series

Seriesn1 (final)Region
Lyman1Ultraviolet
Balmer2Visible
Paschen3Infrared
Brackett4Infrared
Pfund5Infrared

Inner-shell transitions in heavy atoms emit characteristic X-rays; lasers rely on stimulated emission from metastable states.

PMDC learning outcomes for this unit
  • Atomic Spectra: 15.1 Describe and explain atomic spectra / line spectrum

Atomic Spectra MCQs with answers

Practise and track your score →
Q1
Line spectra are emitted by:
AThe sun's surface only
BLiquids
CExcited gases at low pressure
DHot solids
Show answer
Answer: C (Excited gases at low pressure). Isolated atoms of gases emit discrete wavelengths.
Q2
The Lyman series of hydrogen lies in the:
AInfrared region
BUltraviolet region
CMicrowave region
DVisible region
Show answer
Answer: B (Ultraviolet region). Transitions ending at n = 1 involve large energy → UV.
Q3
The energy of an electron in the ground state of hydrogen is:
A0 eV
B−3.4 eV
C−13.6 eV
D+13.6 eV
Show answer
Answer: C (−13.6 eV). E1 = −13.6 eV.
Q4
The energy of the hydrogen electron in the second orbit (n = 2) is:
A−6.8 eV
B−3.4 eV
C−1.51 eV
D−13.6 eV
Show answer
Answer: B (−3.4 eV). E2 = −13.6/4 = −3.4 eV.
Q5
Each element has a unique line spectrum because:
AAtoms have same energy levels
BEnergy levels of each element are different
CAtoms have different masses
DNuclei emit light
Show answer
Answer: B (Energy levels of each element are different). Different energy-level spacings give different photon energies.
Q6
The value of Rydberg constant RH is about:
A1.097 × 10⁷ m⁻¹
B9 × 10⁹ m⁻¹
C3 × 10⁸ m⁻¹
D6.63 × 10⁻³⁴ m⁻¹
Show answer
Answer: A (1.097 × 10⁷ m⁻¹). RH = 1.097 × 10⁷ m⁻¹.
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