Bohr and Rutherford Atomic Models: Key Differences

Bohr's Atomic Model

Rutherford's atomic model was incomplete and unstable. The Danish physicist Bohr proposed a new atomic model based on four postulates, among which was the continuity of orbits:

1. The atom consists of a nucleus with a positive charge and contains most of the mass of the atom, and a cortex in which the electrons are moving.
2. Electrons move in circular orbits around the nucleus.
3. Only orbits in which the electron's angular momentum is a multiple of Planck's constant are possible. In these orbits, the electron does not emit or absorb energy.
4. The step from one orbit to another involves the absorption or emission of radiation. The atom will absorb or emit only the radiation just enough to move from one orbit to another.

The Bohr model can explain the atomic spectrum of hydrogen, but it could not predict the atomic spectra of the remaining elements of the periodic table.

Quantum Numbers

• The principal quantum number (n = 1, 2, 3, 4...), indicates the energy level in which the electron is. This determines the size of the orbital. It takes integer values. It relates to the average distance of the electron from the nucleus of the orbital.
• The angular momentum quantum number, also known as azimuthal (l = 0, 1, 2, 3, 4, 5,..., n-1), indicates the shape of the orbit and the energy sublevel in which the electron is. If:
  • l = 0: Suborbit "s", circular shape
  • l = 1: Suborbit "p", flattened semicircular
  • l = 2: Suborbit "d", lobular, nodal ring
  • l = 3: Suborbit "f", with radial lobar nodes
  • l = 4: Suborbit "g"
  • l = 5: Suborbit "h"
• The magnetic quantum number (m) indicates the spatial orientation of the energy sublevel, "(m = -l,..., 0,..., l)". For each value of l, there are 2l + 1 values of m.
• The spin quantum number (s) indicates the direction of rotation of the magnetic field produced by the electron spinning on its axis. It takes values of 1/2 and -1/2.

Rutherford's Atomic Model

Thomson, the discoverer of the electron, proposed an atomic structure similar to a cake with raisins. The atom was a positively charged spongy area in which electrons were embedded, as many as were necessary to offset their burden and ensure that the atom was electrically neutral. To test this model, Rutherford conducted a series of experiments in which he bombarded a very thin sheet of gold with alpha particles, positively charged radioactive particles. If Thomson's atomic model corresponded with reality, the particles would pass through the atoms without altering their trajectory. Although few, Rutherford found that while most of the particles crossed the plate as predicted by the theory, a few practically bounced in and out. To explain this experience, Rutherford proposed his atomic model, in which the atom is virtually an empty space. In its center is a nucleus, which has almost all the mass of the atom and a positive charge. Electrons revolve around the core, so the whole atom is neutral.