Modeling is an important technique in science. Among other things, it allows us to replicate, examine, and understand things that are either too small or too large for us to see in a normal setting. When it comes to the atom, we can travel all the way back to the ancient Greek philosophers to find mention of the “atomos” or “uncuttable” things in nature but it was John Dalton who gave us the first real scientific theory of the atom- individual elements with defined characteristics that could combine to make other compounds. Dalton’s theory did not include sub-atomic particles, however.
It was about 100 years later when J.J. Thomson realized that atoms have charges, much like a magnet. Thomson’s work led to the discovery of the electron and the realization that the atom contains a positive charge and a negative charge (which he attributed to the electrons). However, Thomson theorized that the atom was a large cloud of positive charge with random points of negative charges scattered throughout. This was later referred to as the “plum pudding” theory (you can imagine a pudding dessert as the positive charge cloud with bits of plum throughout which would be the negative charges).
Most significantly, Thomson’s model of the atom was missing the nucleus which today we know consists of 2 sub-atomic particles. It was only a few years later that Ernest Rutherford built on Thomson’s work and discovered that the atom actually consisted of mostly empty space. Rutherford developed a new model of the atom, one that showed a positively charged center, called the nucleus, surrounded by mostly empty space and some negatively charged electrons.
Just a couple of years later, Niels Bohr modified Rutherford’s atomic model slightly to include electron orbits. In this model, the Bohr model or Bohr diagram model, the nucleus sits in the middle and consists of positively charged protons and neutral neutrons. The electrons circle the nucleus in defined orbits with fixed energy levels. Although we have certainly learned much more about the atom since Bohr proposed his model, it’s this model that I like to use with my middle school students.
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“Although we have certainly learned much more about the atom since Bohr proposed his model, it’s this model that I like to use with my middle school students.”
Since Bohr, Erwin Schrodinger and others have provided a much more thorough and far more complicated understanding of the atom. This has led to an atomic model known as the quantum mechanical model. We now know that electrons don’t really exist in defined orbits, but in orbital clouds, and that they travel in waves rather than a defined path.
The quantum mechanical model of the atom is widely accepted today as the most accurate atomic model. Still, when introducing middle school science students to the structure of the atom for the first time, I revert back to the Bohr model. I like to acknowledge the quantum mechanical model- after all, it’s responsible for the rather cool looking, much more obviously 3-dimensional iconic atom pictures we see all the time. I actually take time to acknowledge, briefly, all 5 of these models that have emerged and evolved through time and explain why we will use the Bohr model- because for our purposes, it is accurate enough. It will give us a basic 2-dimensional drawing or diagram of an atom that will help us understand how atoms bond and form molecules. It’s too simplistic to use with the heavier elements, so it does limit us, but while we are learning the basics, it’s really all we need.