Section 10.1: Atomic Models
Atomic models explain the structure of atoms and the arrangement of subatomic particles. This section covers the historical development of atomic theory and the key models proposed by scientists.
- Dalton’s Model: Atoms are indivisible particles; elements consist of identical atoms; chemical reactions rearrange atoms.
- Thomson’s Model: Atoms contain negatively charged electrons embedded in a positively charged “soup” (plum pudding model).
- Rutherford’s Model: Most of the atom’s mass is concentrated in a tiny nucleus; electrons orbit around the nucleus.
- Bohr Model: Electrons move in quantized orbits; energy is absorbed or emitted when electrons jump between orbits.
Example: Bohr Model Energy Levels
An electron in a hydrogen atom moves from n=3 to n=2. Calculate the energy of the emitted photon using: \[ E = -13.6 \left(\frac{1}{n_f^2} - \frac{1}{n_i^2} \right) \text{ eV} \]
\( E = -13.6 \left(\frac{1}{2^2} - \frac{1}{3^2}\right) = -13.6 (0.25 - 0.111) = -13.6 \times 0.139 \approx -1.89 \text{ eV} \)
The photon emitted has energy 1.89 eV.
Practice Problems
- State Dalton’s postulates and identify one limitation.
- Explain why Thomson’s plum pudding model was later replaced by Rutherford’s model.
- Calculate the wavelength of a photon emitted when an electron drops from n=4 to n=2 in hydrogen.
- Compare the Bohr model and Rutherford model regarding electron arrangement.
- Describe one experimental evidence supporting the nuclear model of the atom.