Alex Fry

Electron Cloud Model of the Atom

The electron cloud, or quantum mechanical, model of the atom, developed by Erwin Schrödinger in 1926, is the currently used and most accurate model for the structure of the atom. At the time that Schrödinger developed this model the accepted model for the structure of the atom was Bohr’s nuclear model, which described the atom as having a central nucleus with electrons surrounding it in perfectly circular concentric orbits. At this time Bohr’s model was being shown inconsistent with experimental data for atoms more complex than hydrogen and so a new, more accurate and consistent atomic model was being searched for.

One scientist, Louis de Broglie, suggested that matter can behave and be described as both a particle and a wave, which was later shown experimentally to be true. This wave-particle duality of matter extends to electrons, which de Broglie modelled as standing waves, rather than particles as in Bohr’s model. This way of modelling the atom, while more accurate, was still flawed as the standing waves representing the electrons were still confined to a fixed space, as standing waves have fixed ends and finite bounds. This presented a problem as Werner Heisenberg had shown that it is impossible to know both the locations and momentum of an electron

simultaneously and that the more accurately you measure on value the less accurately you know the other, in what it known as the Heisenberg Uncertainty Principle. As of this you could only be certain of a probability that an electron would be in a certain place or have a certain momentum at any one time, rather than knowing an exact momentum or location. This caused problems for Bohr and de Broglie’s theories as they both used fixed paths with predefined positions, concentric circles and standing waves respectively, to describe the motion of electrons around the nucleus.

Schrödinger used the work of de Broglie, the wave-particle duality of electrons, and Heisenberg, the Uncertainty Principle and probabilistic nature of electrons, to formulate a new and more accurate model of the electron by constructing complex mathematical formulae to describe the motion of electrons. He stated that electrons move freely in a completely random nature in an area which he called the electron cloud, as it resembled a fuzzy cloud with undefined bounds around the nucleus. The electrons still occupy shells, as in Bohr’s model, each with a different quantised energy level associated with it and electromagnetic radiation absorbed or emitted when electrons transitioned between shells, but instead of fixed paths within them the electrons were able to move freely and randomly within the shells. Each shell is divided into subshells with different shapes and names (s, p, d, and f) which contain electrons with different energies, as described by Pauli’s Exclusion Principle, that no two electrons can exist in the same state with the same quantum numbers. It is within these subshells that the orbitals, the probability maps that represent the possible locations and probability densities of electrons, are found.

Bohr’s Model

Standing waves

de Broglie model

s orbitalp orbital d orbital

f orbital

Alex Fry

If it were possible to stop the motion of an electron within an orbital, marks its exact location, and then repeat for all locations the electron moved to, a three dimensional scatter plot would be built up around the nucleus showing the past and possible locations the electron could occupy. Areas with higher

probability density for an electron to be there are coloured darker, with these areas being closer to the nucleus. This probability map is the cloud-like structure Schrödinger was referring to as the electron cloud.

While this model of atomic structure is currently accepted as the most accurate, it is not always used. This is because Bohr’s model is much simpler and easier to understand. While not every observation can be

explained by it, Bohr’s model can be used to explain many experimentally observed phenomenon and as most chemical processes can be described in terms of the outer shell only it can be very useful. For this reason both the electron cloud/quantum mechanical model and Bohr’s model are used, depending on the nature and needs of the experiments and research being carried out.