The Atomic Models


Scientists cannot work with everything directly. Therefore, chemists, physicists, and biologists need to create models to help describe certain natural phenomena.

But, a scientific model is limited to the information available at the time it is created. So, when new ideas give rise to new experiments that lead to new discoveries the model must be changed or discarded.

Although, scientific models change, models are the only way man has to describe how several natural phenomena work. And knowing how a process functions is often crucial to further scientific discovery.

So, remember models are not truth, models are scientists’ best description of how things work not why things work. For this reason, the importance of models is not their truth, but their workability.

The Greek Model

The idea of matter being made out of small particles is as old as the ancient Greeks. Democritus, in 400 B.C., proposed that all materials were made of small indivisible particles he called atomos.

Although Democritus’ ideas about matter were more accurate, Aristotle’s idea of earth, air, fire and water made more sense to the thinkers of that time.  So, for centuries Aristotle’s ideas were accepted as the correct make up of matter. 

Remember the Greeks never performed any experiments to test the workability of their ideas.

Solid Sphere Model

In 1803 an English teacher, John Dalton, took his ideas and the work of those before him and proposed the first experimental atomic model.

Dalton’s Postulates

  1. All matter consists of tiny, indivisible, indestructible particles called atoms.
  2. Atoms of the same element have the same size, mass and chemical properties.
  3. Differences in the properties of elements result from differences in the atoms of the elements.
  4. The atoms in a compound combine in a definite, simple, whole number ratio.
  5. A chemical reaction is the result of rearrangement, combination, or separation of atoms.

Although, Dalton's model was based on the experimental evidence of others he was the first to present a modern atomic model.

Plum Pudding Model

In 1897 English physicist, J. J. Thomson, suggested that cathode rays consisted of negatively charged particles even smaller than atoms. As a matter of fact he suggested these particles were part of the atom.

Proof of the electron led J.J. Thomson to develop a new atomic theory. He proposed that the atom was a sphere of positive charge with negative electrons placed into it like raisins in a pudding, so he dubbed it the Plum Pudding model.

Nuclear Atomic Model

New Zealand born, Ernest Rutherford assigned his assistants, Hans Geiger and Ernest Marsden, the task of testing the Plum Pudding model. So, Geiger and Marsden in 1909 designed and performed the famous Gold Foil experiment that led to the discoveries of the atomic nucleus and that the atom is mostly space.

Thus, Rutherford proposed that the atom was mostly space with a small, very dense, positively charged nucleus surrounded by electrons in the atomic space. But scientists still couldn’t account for all of the atomic mass. The search was on for a third particle.

Bohr Atomic Model

In 1913 Danish physicist, Neils Bohr, suggested applying Max  quantum theory to the Rutherford model. Bohr's new model proposed that electrons are in fixed energy levels he called orbits. The energy of these orbits is quantized and electrons must absorb or release energy (photons) at certain wavelengths to move between energy levels.

Wave Mechanical Model

In 1926, Erwin Schrödinger combined the Bohr model with Louis de Broglie's hypothesis. He proposed the electron was a 3-D waveform circling the nucleus in a whole number of wavelengths allowing the waveform to repeat itself as a stable standing wave representing the energy levels of the Bohr model.

In support of his hypothesis, Schrödinger developed a mathematical equation to describe the wave-like behavior of the electron. The Schrödinger wave equation not only gave the correct energy levels for the hydrogen atom, but also was somewhat useful in atoms with more than one electron.

Quantum Mechanical Model

In 1928, Max Born applied the uncertainty principle to the Schrödinger equation and suggested that the results of the equation be taken as possible locations for electrons in an atom. These solutions are called quantum numbers.

Electron Arrangements