KEY2CHEM

Comparing Different Atomic Models

The model of the atom is based on experimental evidence. If additional data runs contrary to the current model, the model must be adapted to match the experimental data. This was the case with the classical shell model; the data for atomic spectra of multielectron atoms could not be explained with the classical shell model, so a new model (the quantum mechanical model) was developed to better match the experimental evidence. Previous theories for the structure of the atom (Dalton's "billiard ball" and Thomson's "plum pudding" models are notable examples) were similarly updated based on experimental evidence that could not be explained by the model.


Example 1.

Which experiment disproved Thomson’s “plum pudding” model of the atom?

A. Rutherford’s gold foil experiment

B. Millikan’s oil drop experiment

C. Heisenberg’s microscope thought experiment

 

 

Solution

A. Rutherford’s gold foil experiment

Thomson’s plum pudding model stated that the electrons were distributed throughout the atom. Rutherford’s experiment demonstrated that the positive charge was located in a tiny region near the center of the atom (the nucleus) with the electrons distributed around the nucleus. Millikan's experiment measured the charge on an electron, and Heisenberg's thought experiment proved important points of the Uncertainty Principle, based on classical optics.


Example 2.

Which statement is consistent with the quantum mechanical model of the atom?

A. Electrons circle the nucleus in fixed orbits.

B. Electrons are located in probable regions called orbitals.

C. Electrons are located in a tiny dense core at the center of the atom.

 

Solution

B. Electrons are located in probable regions called orbitals.

The quantum mechanical model of the atom is the most modern theory and states that the location of the electron's matter-wave is most likely to be found in regions of space called orbitals. The classical shell model (also called the Bohr model) used the idea of fixed (or quantized) orbits for the electrons. The nucleus, containing the protons and neutrons, is the tiny dense core at the center of an atom.


Example 3.

Blackbody radiation, the photoelectric effect, and the line spectrum of hydrogen all provided evidence for which model of the atom?

A. Thomson's "plum" pudding model

B. Rutherford's nuclear model

C. Bohr's classical shell model.

 

Solution

C. Bohr's classical shell model

Bohr's classical shell model relied on quantization to explain the experimental evidence. Based on classical physics, a charged electron travelling in a curved path around the nucleus would succumb to coulombic attraction and crash into the nucleus. Since this does not occur, the classical shell model introduced the concept of quantization to hold electrons in fixed levels.