Developing Models of the Atom
- The understanding of what atoms are has changed through time
- Different models have been developed, and then replaced as new evidence from experiments is discovered
- A model is a way of describing something in order to explain the way it behaves
Early Models of the Atom
- Greek and Indian philosophers were the first to try and describe the idea of everything being made up of smaller parts
- The Greek philosopher, Democritus, thought that although objects could be cut into smaller pieces, the smallest possible piece would be indivisible (it could not be cut any further)
- The Greek word for ‘indivisible’ is atomos
- Therefore, atoms were initially thought to be tiny spheres that could not be divided before the discovery of the electron
- Later models described the atom as small solid spheres
JJ Thompson’s Plum Pudding Model
- At the end of the 19th Century, Physicist Joseph Jon Thompson discovered the existence of electrons
- This new evidence meant a better model of the atom was required
- Thompson proposed the Plum Pudding model
- The atom was thought to consist of negatively charged electrons (the ‘plums’) in a positively charged ‘dough’ or ‘pudding’
J J Thomson thought of the atom as being a positively charged mass embedded with small negatively charged electrons – a bit like a plum pudding
- It was known that electrons were much smaller than atoms, so it made sense that they should be embedded within the larger atom
- Since electrons have a negative charge, it was reasoned that the rest of the atom would be positive, making the atom neutral overall
Rutherford's Alpha Particle Scattering Experiment
- In 1909 a group of scientists were investigating the Plum Pudding model
- Physicist, Ernest Rutherford was instructing two of his students, Hans Geiger and Ernest Marsden to carry out the experiment
- They were directing a beam of alpha particles (He2+ ions) at a thin gold foil
- They expected the alpha particles to travel through the gold foil, and maybe change direction a small amount
- Instead, they discovered that :
- Most of the alpha particles passed straight through the foil
- Some of the alpha particles changed direction but continued through the foil
- A few of the alpha particles bounced back off the gold foil
- The bouncing back could not be explained by the Plum Pudding model, so a new model had to be created
When alpha particles are fired at thin gold foil, most of them go straight through, some are deflected and a very small number bounce straight back
- Ernest Rutherford made different conclusions from the findings of the experiment
- The table below describes the findings and conclusions of A, B and C from the image above:
Alpha Scattering Findings and Conclusions Table
The Nuclear Model
- Rutherford proposed the nuclear model of the atom
- In the nuclear model:
- Nearly all of the mass of the atom is concentrated in the centre of the atom (in the nucleus)
- The nucleus is positively charged
- Negatively charged electrons orbit the nucleus at a distance
- Rutherford’s nuclear model replaced the Plum Pudding model
- The nuclear model could explain experimental observations better than the Plum Pudding model
The Nuclear model replaced the Plum Pudding model as it could better explain the observations of Rutherford’s Scattering Experiment
The Bohr Model of the Atom
- In 1913 the Danish Physicist, Niels Bohr, came up with an improved model of the atom
- He used the nuclear model to create his model
- In the Bohr model of the atom:
- Electrons orbit the nucleus at different distances
- The different orbit distances are called energy levels
- Up to 2 electrons orbit in the first energy level
- Up to 8 electrons can orbit in the second energy level
- Up to 8 electrons can orbit in the third energy level
In the Bohr model of the atom electrons orbit in distinct energy levels, which are at different distances from the nucleus
Successes of the Bohr Model
- The Bohr model became the accepted model because:
- It was able to explain the findings from different experiments better than the nuclear model of the atom
- It was able to explain the processes of absorption and emission of electromagnetic radiation
- Theoretical calculations made using the Bohr model agreed with experimental results
The Changing Models of the Atom
- The understanding of the structure of an atom has changed over time
- The best model of an atom is the one that can explain the evidence of experiments best
- As more evidence has been collected, the models have improved
- The image below shows a timeline of the different models of the atom
Scientific models are used to explain observations. Models of the atom have changed and improved throughout history
Worked example
Explain:
a) Why Rutherford's alpha scattering experiment lead to a new model of the atom, called the nuclear model, and replaced the ‘plum pudding’ model.
b) Why it is important that the experimental results and the predictions are the same.
Part (a)
- The experimental results of the gold foil experiment could not be explained using the plum pudding model
- Therefore, the plum pudding model was disapproved and a new model, the nuclear model, was devised to match the results
Part (b)
- If the predictions are correct, then this proves that the nuclear model is correct
Examiner Tip
For the exam you need to be able to describe the features of JJ Thompson’s Plum Pudding model, but you do not need to know how electrons were discovered (this is covered at A Level), or about very early atomic models.Try to remember the different advantages that later models had over earlier models.
