Question

General Chem 1

Compare the current quantum mechanical model of the atom to the classical ( pre-1913 ) model. Identify similarities and at least three differences. Reference to historical development leading to changes may be incorporated.

Answer 1

The first systematic approach for defining an atom was made by Lord Rutherford (1911). The main theme of this model was that a very high concentration of positive charge is present at the central part of the atom. However it suffered from some demerits one of which was explaining the stability of the atom. Niels Bohr's model (1913) however was successful in explaining the stability and line spectra with his model.

The main points in Bohr's model were

1. Electrons in atoms orbit the nucleus (containing protons and neutrons).
2. The electrons can only orbit stably, without radiating, in certain orbits at a certain discrete set of distances from the nucleus. These orbits are associated with definite energies and are also called energy shells or energy levels. In these orbits, the electron's acceleration does not result in radiation and energy loss as required by classical electromagnetics. The Bohr model of an atom was based upon Planck's quantum theory of radiation.
3. Electrons can only gain and lose energy by jumping from one allowed orbit to another, absorbing or emitting electromagnetic radiation with a frequency ν determined by the energy difference of the levels according to the Planck relation:

   Together the above two models were known as Rutherford-Bohr Model or only Bohr model and is considered to be the classical model.

The quantum mechanical model is based on quantum theory, which says matter also has properties associated with waves. According to quantum theory, it’s impossible to know the exact position and momentum of an electron at the same time (Uncertainty Principle).

The quantum mechanical model of the atom uses shapes of orbitals (electron clouds), volumes of space in which there is probability of finding an electron.

Four numbers, called quantum numbers, were introduced to describe the characteristics of electrons and their orbitals:

• Principal quantum number

• Angular momentum quantum number

• Magnetic quantum number

• Spin quantum number

Importantly there are obvious similarities in all the models. Both the Bohr-Rutherford model and Quantum mechanical model agrees on the basic fact that electrons are negatively charged species revolving around the nucleus (positively charged).The Bohr model and the Quantum Mechanical model of the atom both assign specific energies to an electron. The main similarity between the two models is that in both electrons are different distances from the nucleus, corresponding to different energies.

Differences

(a) In the Bohr Model, the electron is treated as a particle in fixed orbits around the nucleus. The energy of an electron corresponds to a specific and fixed distance from the nucleus. In the Quantum Mechanical Model, the electron is treated mathematically as a wave. The electron has properties of both particles and waves. Schrodinger's wave equations are complex mathematical models that describe the energies of the electrons. As with the Bohr model the energies of electrons are quantized, or have only certain allowable values.

(b) The Bohr model was a one-dimensional model that used one quantum number to describe the distribution of electrons in the atom. The only information that was important was the size of the orbit, which was described by “n” the principle quantum number. Schrodinger's model (Quantum Mechanical Model) allowed the electron to occupy three-dimensional space. It therefore required three coordinates. To describe the exact position of the electron, four quantum numbers were used (as mentioned).

Bohr didn't incorporate the idea of shielding in his atomic model. He described only the hydrogen atom, to which the concept of shielding doesn't apply because it has only one electron and one energy level in the ground state. In the QM model, the force of attraction to the nucleus by outer electrons is diminished or "shielded" by the repulsion of electrons closer to the nucleus. Each electron has a shielding factor related to the number of electrons between it and the nucleus. The shielding factor decreases the effective nuclear charge and is related to periodic trends such as atomic size and attraction for electrons.