Evolution of Atomic Models

Atomic Models

Atomic models changed each time a new experiment demanded a better picture. Thomson found the electron and proposed the plum-pudding model, Rutherford’s α-scattering revealed the nucleus, and Bohr explained line spectra with quantized orbits. In the Bohr model the electron energy depends on the principal quantum number n, and light of a fixed wavelength appears when the electron jumps between levels. Here you follow the timeline of models and change n to explore energy levels and transitions.

Why Did Atomic Models Change?

💡 Key: New Experiment → New Model

①Thomson: discovered electron → plum-pudding model (electrons in positive)

②Rutherford: α-scattering → nuclear model (nucleus discovered)

③Bohr: line spectra → orbit model (quantization)

④Modern: electron wave nature → orbital model (probability distribution)

Bohr Model — Energy Levels

Principal Quantum Number n2

Hydrogen Energy Level

E_n = -13.6n² [eV]

Larger n → higher energy (closer to 0)

Light Energy on Transition

ΔE = E_upper - E_lower = hν

h: Planck constant, ν: frequency

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①n=1: ground state → E = -13.6 eV (most stable)

②n=2: first excited → E = -3.4 eV

③Larger n → spacing shrinks

④n→∞ → E→0 (ionization)

Line Spectra and Electron Transitions

Photon Energy

E = hν = hcλ

h = 6.626×10⁻³⁴ J·s, c = 3×10⁸ m/s

💡 Electron Transitions and Light

①Light is emitted when an electron drops from high n to low n

②Larger energy gap → higher frequency (shorter wavelength)

③Hydrogen line spectrum = evidence of transition energies

④Transition to ground state (n=1): UV (Lyman series)

⑤Transition to n=2: visible (Balmer series)

Work It Out

Example 1

In Rutherford's alpha (α) particle scattering experiment, most particles went straight through but a few bounced back at large angles. What atomic structure does this reveal?

Interpret the fact that most particles passed straight through.

Most go straight → the atom is mostly empty space

Interpret the fact that a few scattered at large angles.

Few large-angle scatters → a small, heavy (+) nucleus exists

▸ Discovery of the nucleus (a small, heavy, + charge at the center)

Large-angle scattering is explained only if mass and positive charge concentrate in a tiny core.

Example 2

What phenomenon could the Bohr model explain that the Thomson and Rutherford models could not?

Assume the electron exists only at specific locations, not everywhere.

Electrons exist only at certain energy levels (orbits)

Treat light as emitted or absorbed when an electron moves between levels.

Transitions emit/absorb light of specific frequencies → line spectrum

▸ The line spectrum of the hydrogen atom

A discrete line spectrum appears only if the energy levels are quantized.

Summary

Bohr Energy Level

E_n = -13.6n² [eV]

Energy of an electron in a hydrogen atom

Transition Energy

ΔE = 13.6(1n_f² - 1n_i²) [eV]

Energy change for n_i → n_f

2021 KICE mock Chemistry I type, adapted

Which atomic model first explained the discrete line spectrum of the hydrogen atom?

①Dalton model

②Thomson model

③Rutherford model

④Bohr model

⑤Modern orbital model

▸ ④ Bohr model

The Thomson and Rutherford models cannot explain a line spectrum.

Bohr quantized the electron energy levels to explain the line spectrum.

Bohr: quantized energy levels → explains the discrete line spectrum

🎯 Exam Points

①Order: Thomson → Rutherford → Bohr → Modern

②Bohr limit: only explains hydrogen

③Memorize E_n = -13.6/n²

④Line spectrum = evidence of discrete energy levels

⑤n=1 ground state is most stable (lowest energy)

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Molar Concentration

Electron Configuration

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