I’m sure that at school you have come across the model of the atom below:
You may be so familiar with it that you could label the nucleus, made up of protons and neutrons, with the electron shells orbiting it. However, have you ever asked yourself: How do we know that this is really what an atom looks like?
After all, no telescope nor the human eye can see something as minute as a single atom (around 0.0000000001m in radius).
To get to the bottom of this, we need to look at Ancient Greece which gave us the word for atom from its language in a word called ‘atmos’. They believed that atoms were tiny spheres that made up everything and could not be divided.
In 1897, scientists discovered negative particles inside atoms, tinier than the atom itself. Thus, they disproved thousands of years of incorrectly accepted Ancient Greek theory.
With this new information, a scientist called JJ Thompson developed the ‘plum pudding’ theory of what an atom looks like. Below shows how they thought atoms were solid balls of positive charges with negative charges sprinkled in, like a traditional English Christmas pudding.
See the similarity?
This is the model that was accepted for a while, but as we know, it wasn’t the correct one.
In 1909, a scientist called Ernest Rutherford designed an experiment to test the plum pudding model. In the experiment, positively charged alpha (positive) particles were fired at thin gold foil. Most alpha particles went straight through the foil. But a few were scattered in different directions. That is why this experiment was called the ‘Gold Foil Experiment’ or the ‘Alpha Scattering Experiment’.
Rutherford learnt from this experiment and observed that the very small number of alpha particles must mean that there is a dense centre of positivity in the centre of the atom - called a nucleus. The fact that so many alpha particles went straight through told Rutherford that there must be huge gaps.
Niels Bohr was a scientist who, in 1913, adapted Ernest Rutherford's nuclear model. Bohr calculated that electrons orbit the nucleus in shells. He proved that these shells are at certain distances from the nucleus. The calculations agreed with observations from experiments.
Additionally, more experiments proved that there were protons within an atom.
In 1932 James Chadwick proved the existence of particles in the nucleus with mass but no charge. These particles are called neutrons. This led to another development of the atomic model, which is still used today.
This atomic model includes protons and neutrons in a nucleus, with electrons orbiting it in electron shells, like below.
By: Hemlata Pant
Sources:
Bertsch McGrayne, Sharon. “Atom | Definition, History, & Examples.” Encyclopedia Britannica, www.britannica.com/science/atom. Accessed 24 July 2020.
“GCSE Science Revision Chemistry ‘Alpha-Scattering Experiment.’” YouTube, uploaded by Shaun, 23 Oct. 2016, www.youtube.com/watch?v=KEuAcoCai_E.
“GCSE Science Revision Chemistry ‘The Nuclear Model.’” YouTube, uploaded by Freesciencelessons, 23 Oct. 2016, www.youtube.com/watch?v=Yf-WhHZuqes.
“Structure of the Atom - Atomic Structure - AQA - GCSE Combined Science Revision - AQA Trilogy.” BBC Bitesize, BBC Bitesize, www.bbc.co.uk/bitesize/guides/zwn8b82/revision/3. Accessed 24 July 2020.
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