In 1904, New Zealand scientist Ernest Rutherford conducted an experiment that became known in the history of science as the Rutherford experiment.
He already knew of the existence of positively charged particles called alpha particles ( ). In his experiment, Rutherford placed a particle-emitting substance inside a lead block so that they were directed through a hole in a lead plate to collide with a thin sheet of gold.
Observe the scheme of Rutherford's experiment:
With this experiment, he found that:
- most particles crosses the golden blade;
- some of these particles as they crossed the blade were deflected;
- a small part of the particles it does not cross the blade and returns, as if these particles collide with something very dense.
Looking at these results, Rutherford concluded that:
- The atom is not a massive sphere. There are large voids as most particles crossed the golden blade.
- The atom has a central region where its mass is concentrated. It was against this region, which he called the nucleus, that the particles , shocked and returned.
- This nucleus has a positive charge because it repelled the particle - which also has positive charge.
With this data, Rutherford has built an atomic model similar to the Solar System, where the atom is a very small particle composed of two regions:
- an internal one, the nucleus, where practically all the mass of the atom would be concentrated - of positive electric charge, represented by particles called protons;
- another external, of negligible mass, where would be the electrons, tiny negative particles moving around the nucleus.
In the models presented here, the core and electrosphere dimensions are not to scale. In fact, the electrosphere is about 100,000 times larger in volume than the nucleus.
In 1932, the English physicist James Chadwick (1891-1974), experimenting with radioactive material, proved one of Rutherford's hypotheses that the possibility of having another particle without an electric charge exists in the nucleus.
Chadwick called these particles neutrons.