The basic building block of Chemistry is the atom it is therefore useful to define what an atom actually is.

Atoms are made up of positive (protons), negative (electrons) and neutral (neutrons) subatomic particles. Physics further breaks these particles down into fundamental particles that are not discussed here. Instead, we start here with our study of chemistry – the discovery and the modern definition of the atom.

Democritus – 450 B.C. – coined the name atomos meaning “uncuttable” and defined it as the term for “the smallest indivisible particle of matter.”

1. John Dalton Billiard Ball model – 1808 – In this model, atoms are seen as single, indivisible spheres. Molecules or chemical compounds were defined as groups of atoms arranged in a specific ratio.   fivemodelsoftheatom-billiardball
Problems with this interpretation – it was later discovered the atom contained positive protons (p+) and negative electrons (e). Protons are much heavier than electrons and the presence of the two made atoms electrically neutral. Note: it is necessary that you know that opposite charges attract and similar charges repel.
 2. J. J. Thomson – Raisin Bun Model – 1904 – Joseph John Thomson and coworkers correctly characterized the electron as a separate type of negative particle using cathode ray tubes. This led to the Raisin Bun Model of the atom where it was believed the negatively charged electrons were embedded in a large positive cloud that made up the atom.   fivemodelsoftheatom-jjthomson
3. Ernest Rutherford – The Nuclear Model – 1909 – attempted to prove Thomson’s theory by shooting alpha particles (Helium atoms) through a gold foil. He expected that the alpha particles would go through the gold foil unimpeded but instead a small fraction of the alpha particles rebounded back at the source of alpha radiation. This led Ernest Rutherford to believe that there was a large nucleus from which the alpha particles rebounded. The Nuclear Model or Empty Space Model was born. The atom has a central core that is positively charged. Electrons exist in empty space, surrounding this central positive mass, travelling anywhere they want.  fivemodelsoftheatom-rutherford
Problems with this interpretation – It was thought the electrons would collapse towards the nucleus destroying the atom and that they would continually release electromagnetic energy as they spiraled towards the nucleus.
4. Niels Bohr – Energy Levels – 1913 – Bohr combined Max Planck’s theories of quantum mechanics and Rutherford’s model to define a model where the core of the atom was the nucleus, containing neutral particles called neutrons and positive particles called protons.  Electrons had different defined amounts of energy, therefore had to exist at specified distances from the nucleus, in orbits. It was when the electron was found in these orbitals that it was stable and would not travel towards the nucleus as predicted by classical mechanics (classical physics). This meant that the electron was quantized or had only a certain quantity of energy when found at different energy levels. fivemodelsoftheatom-nielsbohr
Problems with this interpretation – similar to the problems with Rutherford’s model AND this model only worked for single electron atoms (like hydrogen).
5. Quantum Mechanical Model – 1920s and Onwards -– Cloud Model – this is not necessary to know in detail for Alberta Curriculum high school students.

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