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Development of the model of the atom
Development of the model of the atom
Before the discovery of the electron the atom was thought to be a sphere that could not be divided any further.
Plum pudding model
In 1897 the discovery of the electron by J.J.Thomson led to the plum pudding model. He carried out investigations where he found that the surface of hot metals emitted electrons. The plum pudding model is described as a ball of positive charge with negative electrons embedded in it.
Alpha particle scattering experiment
Results from the scattering experiment by Rutherford at Manchester university in 1911 led to the conclusion that the mass of the atom was concentrated at the centre (nucleus) and that the nucleus was charged.
This model, called the nuclear model, then replaced the plum pudding model.
Alpha Particle Scattering
Between 1909 and 1911, Geiger and Marsden, students of Lord Rutherford at Manchester University, undertook an experiment in which they aimed an alpha particle source at an extremely thin gold foil.
Results of the alpha particle scattering experiment
Empty space
- Alpha particles, positively charged helium nuclei made of two protons and two neutrons, are fired at a thin layer of gold foil
- The alpha particles are fired in a thin stream at the foil
- The source producing the alpha particles is radium as it has high activity producing lots of particles
- The results of the experiment showed that most of the alpha particles passed straight through the gold foil, even though the alpha particles are being fired at a solid surface
- This shows that atoms are actually mostly empty space
Dense positive nucleus
- Some of the alpha particles were deflected slightly, suggesting that there must be a small part of the gold atom that is positively charged
- This is because the alpha particles are positive and like charges repel each other, so the positive part of the nucleus deflected the alpha particles
- A small number of nuclei were deflected by more than 90 degrees so the positive part must be very dense
- The small, dense, positive centre was named the nucleus
- The Rutherford experiment shows us that an atom has a small positive nucleus
Nuclear model
Bohr’s model
Arrangement of electrons
Another scientist called Niels Bohr in 1913 found that electrons occurred at specific distances from the nucleus so the model was further adapted.
His calculations agreed with experimental observations.
We describe these specific distances where the electrons are found as shells or energy levels.
Niel's Bohr model, electrons arranged in energy levels (shells)
Protons
Later experimental work found that the positive charge in the nucleus could be subdivided into whole number of smaller particles. The particles had the same positive charge and were described as ‘protons’.
Neutrons
Another particle was discovered in 1931 by the experimental work of James Chadwick. This was the uncharged (neutral) sub atomic particle called a neutron found in the nucleus.
Exercise 1
State, using 3 - 5 words, what was discovered by the following scientists during the development of the model of the atom (where possible, state the model):
- JJ Thomson 1897
- Ernest Rutherford 1911
- Neils Bohr 1913
- Further work
- James Chadwick 1931
