seegongsik
Grade 10-11 (age 15-17)

high school Evolution of Atomic Models

Atomic models have changed each time a new experimental result appeared that the old model could not explain. Thomson discovered the electron and proposed electrons embedded in positive charge, Rutherford's alpha-particle scattering experiment led to a model with a nucleus, and Bohr proposed an orbit model that explained the line spectrum of hydrogen. In the Bohr model the electron's energy takes only separate values set by the principal quantum number n, and when the electron drops from a higher level to a lower one, light carrying the energy difference comes out. In the picture, changing the principal quantum number n changes the electron's orbit, its energy level and the energy of the light given off when it falls to n = 1.

Why Did Atomic Models Change?

💡 Models Changed to Fit New Experiments
①Thomson: discovered the electron and proposed a model with electrons embedded in a lump of positive charge
②Rutherford: the alpha-particle scattering experiment revealed a small, heavy nucleus at the center of the atom
③Bohr: to explain hydrogen's line spectrum, proposed that electrons move only in fixed orbits
④Modern: to reflect the wave nature of the electron, uses an orbital model showing the probability of finding the electron
⑤The Bohr model explains only one-electron atoms such as hydrogen well and cannot explain atoms with many electrons

The Bohr Model and Energy Levels

2
Energy Levels of the Hydrogen Atom
En = −13.6n2 [eV]
The larger n is, the higher the energy, approaching 0
Energy of the Light Emitted in a Transition
ΔE = Eupper - Elower = hν
h is the Planck constant and ν is the frequency of the light
🔍 Reading the Picture
①n = 1 is the ground state, E = −13.6 eV, the most stable
②n = 2 is the first excited state, E = −3.4 eV
③As n grows, the orbit grows in proportion to n², and the gaps between energy levels get smaller
④As n → ∞, E → 0 and the electron leaves the atom (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
①When an electron moves from a higher level to a lower one, it gives off light carrying the energy difference
②The larger the energy difference, the higher the frequency and the shorter the wavelength of the light
③Hydrogen's line spectrum is evidence that the energy levels are spaced apart
④Transitions down to n = 1 give ultraviolet light (Lyman series)
⑤Transitions down to n = 2 give visible light (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?
1
Interpret the fact that most particles passed straight through.
Most go straight → the atom is mostly empty space
2
Interpret the fact that a few particles bounced back at large angles.
A few scatter at large angles → a small, heavy (+) nucleus exists
▸ The discovery of a small, heavy (+) nucleus at the center of the atom
Only if the atom's mass and positive charge are packed into a tiny center can alpha particles bounce back at large angles.
Example 2
What phenomenon could the Bohr model explain that the Thomson and Rutherford models could not?
1
Assume that electrons are not just anywhere but only in fixed orbits.
Electrons exist only at fixed energy levels (orbits)
2
Take it that electrons give off or absorb light when they move between levels.
Transitions between levels → light of fixed frequencies emitted or absorbed → line spectrum
▸ The line spectrum of the hydrogen atom
Only if the energy levels are fixed and spaced apart does light come out with fixed energies, making the line spectrum discrete.

Summary

Energy Levels in the Bohr Model
En = −13.6n2 [eV]
The energy of the electron in a hydrogen atom
Transition Energy
ΔE = 13.6(1nf2 - 1ni2) [eV]
The energy change in a transition from ni to nf
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
1
The Thomson and Rutherford models cannot explain a line spectrum.
2
Bohr explained the line spectrum by quantizing the electron's energy levels.
Bohr: quantized energy levels → explains the discrete line spectrum
🎯 Exam Points
①Order of atomic models: Thomson → Rutherford → Bohr → modern model
②The Bohr model can explain only the one-electron hydrogen atom
③Energy levels of hydrogen: Eₙ = −13.6/n² eV
④Line spectra are evidence that energy levels are discrete
⑤The ground state n = 1 has the lowest energy and is the most stable
← Previous
Molar Concentration
Next →
Electron Configuration

Last updated

Was this helpful? Support seegongsik